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.

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 rurl URL shortening, clickable links and tweets, customization, and both public and friend timelines.

Chirrup - Twitter client with extensive Japanese language support.

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.

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.

Tweetr - Submit tweets, upload files, urltea built in, delete messages.

Spaz - Cross-platform client built with AIR.

Tweeter - Java based client with customizable UI.

Twinja - AIR based client which allows you to follow users on the fly.

Twitterverse - Search through archived public timelines and tweets.

Twittersearch - Search Twitter and tweets by word.

Terraminds - Search for specific users or tweets.

TwitDir - Allows user to search for users by name, location, or username.

Browser Plugins

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.

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.

Mobile

iTweet - A mobile Twitter site designed for the iPhone.

iTwit - Mobile version of Twitter made for the iPhone.

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.

jTwitter - View timelines, enter updates, and more, all on your mobile phone.

TreoTwit - Easily check and update your Twitter right from your Treo.

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.

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.

Information below sourced from http://www.ask-kalena.com/ with Kalena's Permission

Retweet This - Is an application designed for use with the Greasemonkey Firefox plug-in. It allows you to append a retweet button to the end of each Twitter entry to encourage your followers to retweet your posts.

Classify Twitter Users is a script that allows you to classify Twitter users and decide whether they are worth following based on their friends, followers and post ratios. Another Greasemonkey app, it’s a great tool to
help you weed out the Twitter spammers and fan boys from the socially healthy.

You can set the script to use your own Twitter user scales (e.g. spammer, social climber, social rock star) or just gather more information about people you are following.

TwittAd allows Twitter users to monetize their account by accepting advertising on the background image of their Twitter user profile. You decide the duration and price advertisers pay for exposure and get paid for
every hour you serve the ad.

TwitterCounter lets you add a daily updating TwitterCounter to your blog so everybody can see how popular you are by the number of persons following you.

TweetBeep is just like having Google Alerts for Twitter. It enables you to keep track of conversations that mention you, your products or your company via email. You can even keep track of who’s tweeting your
website or blog. It’s a great tool for online reputation management and you don’t even have to be a Twitter user to benefit from it. TweetBeep is another one of my personal *must haves*.

Twitter Grader measures the reach and authority of a Twitter user, calculated by the pace of their updates, the completeness of their profile, their number of followers and the network power of those followers.

Twitter Grader displays as a score out of 100 and is consistently updating and adapting as your Twitter account grows. Based on those grades, Twitter Grader lists the Twitter Elite globally and in each country,
just like Technorati does for bloggers.

TwitPic, as the name suggests, is a photo sharing tool for Twitter. When logged in, it allows you to upload photos and post them with comments as a Tweet. It works with a range of Twitter clients such as Twitterific,
Twhirl and MobileTwitter and stores all your photos in a single location with updated viewing stats. As far as Twitter image sharing goes, this is king for reliability.

Twuffer is a Twitter buffer. It allows Twitter users to compose a list of future tweets, and schedule their release. Twuffer is ideal for making hourly, daily or monthly announcements or send post-dated birthday greetings
or reminders to people about upcoming events. It’s also perfect if you’re the type of person who wants to give the impression that you never sleep.

Twistori I have a real soft spot for. One of the developers is Amy Hoy, one of my favorite speakers from Webstock 2008 and a goth geek to boot. Twistori is a social experiment that taps into the Twitter conversations
revolving around life’s central activities: Love, Hate, Think, Believe, Feel, Wish. You can click on any of these emotions and witness a live feed of Tweets that use them. It’s voyeuristic and utterly compelling.

iTweet is another interface for Twitter. It has built-in auto-refresh, search and hash tags and features full follow, block and notifications features. Users can view and post user bio, location and URLs inline with their tweets.

Another cool feature of iTweet is the ability to Retweet a user post with a single click.

Twitturly is a service for tracking what URLs people are talking about as they talk about them on Twitter. Similar to Digg, on Twitturly, people “vote” for a URL. The more votes it gets the better it ranks.

If it does well enough, it gets promoted to the home page and as the votes increase it gets displayed higher up the home page. Twitturly differs from Digg in that instead of voting on their site, you vote by participating on Twitter.
Each time that you send a link to your followers on Twitter, Twitturly takes a note of it and applies your vote to that URL. It’s a great way to follow the loudest global conversations.

Mr Tweet is another big favorite of mine. Similar to LinkedIn, it looks through your extended network and makes suggestions to help you build effective relationships on Twitter. For example, which of your followers should
you be following in return? Who are the most influential people you should be following? Who are your followers following?

Twitter Search is Twitter’s own built-in and oft-overlooked search tool. You can use it to search for other Twitter users, keywords, hash tag topics and a range of cool shortcut items.

Non Twitter Specific Tools:

FlipTitle is, not surprisingly, a tool that enables you to flip text upside down. It’s great for Twitter because sometimes your tweets can get lost in the conversation. Using FlipTitle makes sure they get noticed.

Bit.ly is a very cool URL shortener that also includes click-thru statistics. Why is it perfect for Twitter? Because the length of the converted URLs is generally much shorter than other URL converters like TinyURL.

When you are tweeting, space is premium as your whole post can only consist of 140 characters including spacing.

The other neat thing aspect of Bit.ly is that it keeps a record of clicks that your URLs received and where they originated from. It even provides a share button so you can re-tweet your URLs on Twitter or send them to
friends on Facebook, Gmail or another email client.

Must Have Tools & Apps