Twitter Desktop Applications

TweetDeck
TweetDeck is a desktop application that lets you organize your follows into groups and tracks your @replies and direct messages.

http://www.tweetdeck.com/beta/

Seesmic & Twhirl
Similar to TweetDeck, Seesmic and Twhirl are desktop applications that also allow you to manage multiple accounts, so you can separate your work and personal accounts, for example.
http://twhirl.org, http://seesmic.com

GroupTweet
If you’re using Twitter for internal communication and want to keep messages private, this app lets you broadcast Tweets only to a group of specified users.

http://www.grouptweet.com/

gTwitter
If you use Linux, there’s a desktop client for you, too. http://code.google.com/p/gtwitter/

TwitterMail, TwitterIM, and OutTwit
Tweet from your email, IM and Microsoft Outlook. http://www.twittermail.com/, http://www.twitterim.com, http://www.outtwit.com

Twitter Photo and Video

TwitPic
Share photos with your friends on twitter with twitpic. No signup required, just login using your twitter account. TwitPic is the most popular, but you can also try these other photo sharing services: Twitxr and 2Tweet. http://twitpic.com, http://www.twitxr.com, http://2tweet.com/

Twiddeo
Twiddeo is a powerful but simple service that let’s you do one thing very well: Twitter updates with Video. Upload from the web, your cameraphone and record from your webcam.

http://beta.twiddeo.com

MobyPicture
Directly share photos with all popular social sites: facebook, twitter, flickr, blogger, vimeo, and more!

http://mobypicture.com

TweeTube
Share your favorite videos quickly with short URLs. TweeTube lets you track the visits and comments from the people that follow you on Twitter.

http://www.tweetube.com/

Twitter On Your Smartphone

Tweetie
Tweetie is a fast, full-featured twitter client for iPhone and iPod touch.

http://www.atebits.com/software/tweetie/

PocketTweets
PocketTweets is a Web-based Twitter client for the Apple iPhone. See the latest tweets from your contacts, update your status remotely, or see what’s happening around the world by viewing the public timeline.

http://pockettweets.com/

Twitterberry
TwitterBerry is mobile client for posting updates to Twitter. It works over the data network, so you don’t need to use SMS.

http://www.orangatame.com/products/twitterberry/

jibjib
jibjib is another open source J2ME Twitter client designed for all twitter-ers on the road. One hand controls the wheel and another hand tweets along the road. http://sugree.com/project/jibjib

Twitterfon
Another Twitter app for the iPhone. This one is free! http://www.twitterfon.com/

Twitter Analytics

Twitalyzer
Plug in your username for quick statistics on your clout, influence, and the quality of your tweets.

http://twitalyzer.com/twitalyzer/

Tweetburner
Track the links you share to see if your followers are interested.

http://tweetburner.com/

Twit(url)ly and Twitmeme
Find the most popular links on Twitter.
http://twitturly.com/, http://www.twitmeme.com

TweetStats
Graph your Twitter statistics.

http://tweetstats.com/

Twitterholic
Find popular Twitter users (and check out their methods).

http://twitterholic.com/

Twitter Fun Stuff

The Twictionary
From Attwaction to Twoosh, keep up on all the Twitter lingo, both common and obscure.

http://twictionary.pbwiki.com/

Who Should I Follow?
Get personalized recommendations to find new friends. http://whoshouldifollow.com

Twittercal
Automatically add events to your Google Calendar by DMing them to @gcal. http://twittercal.com/

TwitterFone
Send tweets via voice. http://www.twitterfone.com/

LOLWRUS
Combine tweets and Flickr images to create funny photos (similar to LOLCATS).

http://lolwr.us/

Twitstori
A beautiful real-time stream of users tweets.

http://twistori.com

Twittearth
See tweets from all around the globe.

http://twittearth.com/