Twitter has become one of the ultimate tools for communication and business. You can use twitter for almost everything. It is a great service but it would not have been so popular if there were no apps or plugins. There are tons of apps to help you get the most out of twitter. I have compiled a list of the most important and valuable twitter apps and plugins to help you achieve your Twitter goals. I am highlighting more than 100 of my favorite and useful twitter add-ons.
Apps, Widgets and Plugins for Windows
TwitBox – View and submit tweets, see replies and direct messages, delete your own tweets, and multiple account support.
Twitterlicious – Update Twitter status, easily check replies and direct messages.
Twitteroo – Client with URL shortening, clickable links and tweets, customization, and both public and friend timelines.
KipFolio Widget – Simple Twitter widget for KipFolio.
Twitter-Sync for Yahoo – Syncs Twitter status with Yahoo Messenger status.
TwitterYM – Simple Yahoo Messenger status updater.
Twessenger – Updates Live Messenger status to reflect latest tweet.
Twit4Live – Set status message as latest tweet, send tweets from within Live Messenger, and send tweets to contacts.
TwittIt – Extremely compact and simple app for submitting tweets.
MadTwitter – A reincarnation of Twitterrific, except on Windows.
Twitter CLI – Post tweets via the command line.
What’s Up? – Gadget that allows you to see latest tweets from your friends.
Twadget – Simple gadget that lets you view and submit tweets right from Vista’s Sidebar.
Triqqr – Submit tweets directly to Twitter and view user profiles.
Pwytter – Cross platform Python Twitter client with Asian character support.
Teletwitter – Experimental open source client.
Apps, Widgets and Plugins – Mac
TweetAhead – Schedule tweets to be posted ahead of time with a simple Dashboard widget.
Twitterlex – Dashboard widget that shows your friend’s latest tweets as well as allows you to submit tweets.
Twidget – Update your Twitter status within Dashboard.
Twitterrific – A compact client that publishes tweets, shows unread tweets, has easy access to profile pages, shows either public or friend timeline, and more.
TwitterPost – Basic client that submits tweets, shows your current iTunes track in your feed, and more.
Twitgit – Read your friend’s tweets and submit your own, right in Dashboard.
TikiTwit – Match your iChat status to your last tweet.
Apps, Widgets and Plugins – Linux
ZenTwitter – A GUI frontend for Twitter using zenity and curl.
Twitux – Twitter client for the GNOME desktop.
gTwitter – GTK+ based app inspired by Twitterrific.
BLT – Perl script that lets you see what your friends are doing right from the Terminal.
Twitter for Vim – Publish tweets right within Vim.
Twitter.el – Publish tweets while in Emacs.
Twitbar – Post tweets from the GNOME deskbar.
Twit.el – Another utility that allows publishing of tweets within Emacs.
Twitter for Blogging
Add to Any– Get your posts shared on Twitter by using Add to Any on your WordPress blog.
Twitpress– This will send out a Tweet every time you post a new blog entry.
TwitterCounter– You can display the number of followers you have on Twitter.
TwitterFeed– Announce your blog post on Twitter with a customized message using TwitterFeed.
TwitThis– Make use of this plugin to send Twitter messages about your blog post.
MyTwitter– This will display your Twitter status on WordPress.
Terraminds – Search for specific users or tweets.
TwitDir – Allows user to search for users by name, location or username.
Twitterverse – Search through archived public timelines and tweets.
Twittersearch – Search Twitter and tweets by word.
Twitter Friends– Carefully measure your Twitter conversations using Twitter Friends.
Twinfluence– This tool will measure your Twitter influence based on reach, velocity, and social capital.
Twitter Grader– Learn your Twitter grade, your local Twitter Elite, and find new people to follow through Twitter Grader.
Twitterholic– Check out the top Twitter users and find out your Twitter stats on Twitterholic.
TweetStats– This tool offers a graphical analysis of your Twitter stats.
Tweet-Rank– Learn about the quality of your tweets by finding out which ones won or lost followers.
Mr. Milestone– Get a tweet when you reach various milestones using this Twitter tool.
Retweetrank– Find out how many retweets you and other Twitter users have through this service.
Tweetwasters– Find out how much time you and other users waste on Twitter.
Twitter Information Gathering
Tweetbeep– You can set up alerts that will help you keep track of keywords on Twitter.
Twitscoop– Twitscoop shares what’s hot on Twitter at any given moment.
Twitbuzz– This tracks the latest conversations as well as popular Twitter links.
StrawPoll– Use StrawPoll to make sharing your opinion easy.
TweetNews– TweetNews ranks stories based on the amount of related tweets.
TwitterBuzz– TwitterBuzz will tell you what’s being linked to the most on Twitter.
Tweetscan– Set up Tweetscan to make sure you don’t miss any @replies, and to get alerted of your search queries.
@myflightinfo– Keeps you updated on your flight’s status.
Twitterverse– Check out archived timelines and tweets through Twitterverse.
Retweetist– This service ranks the hottest links being retweeted on Twitter.
Monitter– Get real time keyword monitoring on Twitter from Monitter.
Twitter Network Building & Management
Twitter Local– With this service, you can see tweets from Twitter users in a specific location.
TwitDir– With TwitDir, you’ll be able to search for people, and exploring categories including top followers and updaters.
Who Should I Follow? – Using this site, you can get good recommendations for Tweeps to follow.
Nearbytweets– Learn about all of the Twitter users in a specific area with the help of Nearbytweets.
Twellow– Find Twitter users in a specific industry using this service.
Mr. Tweet– Mr. Tweet is a personal networking assistant for Twitter, helping you find relevant followers.
Twubble– This will expand your Twitter bubble, picking out people you may like to follow.
MyCleenr– Sort your friends by their last tweets, and you can get rid of the inactive and useless accounts you’re following.
Follow Cost– This tool will tell you how much effort it takes to follow someone.
Tweepler– This helps to organize your tweeps based on whether you’re following them or not.
Just Tweet It– Find Tweeple, tools, Twitter bots and more through this directory.
TweetWheel– TweetWheel will help you discover which of your Twitter friends know each other.
SocialToo– SocialToo will help you keep track of all of the people who have followed or unfollowed you.
Twitoria– Reduce your clutter on Twitter by finding your friends that haven’t tweeted in a long time.
Qwitter– Qwitter will help you manage your network by sending an alert when a person unfollows you.
Summize– Retrieve information on Twitter quickly to search Twitter in real time.
Tweet O’Clock– Trying to reach someone? Tweet O’Clock will help you find the best time to get their attention.
Just Signal– Set up a filter using Just Signal to get only the tweets that discuss the keywords you’d like to read about.
TweepSearch– Put your Twitter network to good use and search your followers for specific parameters.
Friend or Follow– Manage your Twitter contacts and find out who’s not following you back through Friend or Follow.
TwitResponse– TwitResponse makes it easy for you to schedule the delivery of your tweets ahead of time.
TwitterSnooze– Put the pause button on a particular user for a while with Twitter Snooze.
Twitterless– Get notified when someone stops following you with Twitterless.
Twilert– Track specific keywords to receive alerts for using Twilert.
Tweetdeck– Tweetdeck has a groups function that will help you more efficiently follow the people you really want to listen to.
Twalala– Put the mute button on certain people and topics for a while if you are receiving lots of updates you’re not really interested in.
Twitter File Sharing Tools
Ping.fm– This service will update all of your social networks at once.
TweeTube– TweeTube makes it easy to share videos on Twitter.
twiggit– Use this automated service to share the articles you digg on Twitter.
Twisten.fm– Share what you’re listening to on Twitter through Twisten.fm.
Tweetburner– Use Tweetburner to share links, and you can track their usage.
Twitpic– Twitpic makes it easy to take mobile phone photos and share them using your Twitter account.
TwitterHawk– Get targeted marketing on Twitter through TwitterHawk.
Acamin– Acamin makes it easy to share files on Twitter with your followers.
Glue– Post links to books, movies, restaurants and more on Twitter through Glue.
Tweet Later– Tweet Later offers a great way to set up alerts, schedule tweets, send thank you DMs, and more.
OutTwit– OutTwit will make it easy for you to use Twitter inside of Outlook.
Jott– Jott makes it easy for you to tweet without ever having to type, transcribing your voice message to Twitter.
Twittercal– Link your Twitter account and Google Calendar to easily keep up with your events and appointments.
Timer– Use Timer to get reminders about tasks through your Twitter account.
TwitterNotes– Organize your notes using Twitter with TwitterNotes.
Remember the Milk– Use This on Twitter to update your to do list.
Tweetake– Tweetake will back up your Twitter timeline for archiving and more.
Nozbe– Nozbe makes it easy to add and update your to do list on Twitter.
Toodledo– This popular to do list app integrates nicely with Twitter.
TrackThis– Send TrackThis your tracking number, and you’ll get Twitter messages every time there’s a change in location.
Joint Contact– Get project management productivity on Twitter using Joint Contact.
Tempo– This time tracking tool allows you to send in updates from Twitter.
Twitter for Business & Finance
CheapTweet– Get all of the deals, sales, coupons and more being discussed on Twitter through CheapTweet.
Tipjoy– Like Chipin, Tipjoy offers a way to create social payments for your cause, content, or people.
SalesTwit– Get contact management for Twitter with the help of SalesTwit.
Tweet What You Spend– Track your cash in a really effective way using Tweet What You Spend.
StockTwits– StockTwits shares the investment discussions on Twitter in real time.
Chipin– If you are raising funds on Twitter, make use of this incredible tool to set a goal and let your supporters track the progress of the campaign.
Xpenser– You can Twitter your expenses to Xpenser and they will be recorded for you.
Twittertise– Schedule your tweets and track their clickthroughs with this app designed for Twitter advertising.
TwtQpon– Create simple Twitter coupons for your business with TwtQpon.
Twitter Browser Plugins
Search Plugin – Tweet right from the Firefox search box.
Tweetbar – Put Twitter on your Firefox sidebar.
Twippera – Twitter widget for Opera that can send and view tweets.
TwitterFox – Compact Firefox extension that can publish and view tweets.
TwitBin – Send and view tweets right within Firefox.
TwittyTunes – A FoxyTunes addon that posts what you’re currently listening to Twitter.
Shareaholic – Share links and webpages via Twitter right within Firefox.
Twitter for Mobile
Tweeter – Twitter on your mobile phone while avoiding text message charges.
Jargong – Browse Flickr, do some social networking, but most importantly, check and publish tweets.
WildSets – Extensve Twitter app for mobile phones.
TwitterBerry – Twitter built for various BlackBerry devices. Update and view timelines with ease.
iTweet – A mobile Twitter site designed for the iPhone.
iTwit – Mobile version of Twitter made for the iPhone.
jTwitter – View timelines, enter updates, and more, all on your mobile phone.
TreoTwit – Easily check and update your Twitter right from your Treo.
TwitterNotes – Share private or public notes via Twitter.
RSS2Twitter – Share any RSS feed on Twitter, and have new RSS items automatically be tweeted.
TwitterBuzz – Find out what is being most linked to on Twitter.
Twitterholic – Shows the top users and accounts on Twitter.
Twittervision – See where in the world people are submitting tweets from, real time.
TwitThis – Allow visitors to your site to easily share a page or post via Twitter.
MoniTwitter – Receive important updates about your site via Twitter.
I hope you find this useful. I have personally verified most of the apps/plugins and they are good. It took me almost 5 hours to go through them . Last but not the least, I would like to thank mashable & techcrunch for making my find easier.