Twitter

BrightKit
Enables Twitter users to manage multiple Twitter accounts and pre-schedule tweets
http://www.brightkit.com

Commuter Feed
Get traffic and transit updates from Twitter posts.
http://www.commuterfeed.com/

FutureTweets
A free service that lets you schedule your Twitter messages at a specific time in the future or send a reoccuring Tweet daily, weekly, monthly or yearly
http://futuretweets.com

Just Tweet It
Twitter directory helps you find Twitter users classified by interest and occupation. Also can browse Twitter tools, services and latest news
http://justtweetit.com

PollDaddy Twitter Polls
Enables the easy creation of hosted polls for Twitter
http://twitter.polldaddy.com/

SayTweet
Create a mashup of pictures with Twitter messages. Upload a picture, tag Twitter users in the picture, then embed the dynamic badge on any web site or blog.
http://saytweet.com

The Shorty Awards
Honors the world's top Twitterers for their ability to write great content with 140 characters or less.
http://shortyawards.com/

Tweet Scan
Real-time search engine for Twitter and identi.ca.
http://www.tweetscan.com/

TweetBeep
Enables anyone to receive alerts by email whenever a specific word or phrase is tweeted on Twitter.
http://tweetbeep.com/

TweetLater
Schedule future-dated Twitter tweets
http://www.tweetlater.com

Twellow
A search directory of people by area of expertise, profession or other attribute listed in personal profiles on Twitter.
http://www.twellow.com/

Twiddeo
Offers the option to add video to a message.
http://www.twiddeo.com/

Twistori
The first step in an ongoing social experiment based on Twitter. Provides a dynamic stream of Tweets that include words such as love, hate, think and believe.
http://twistori.com

Twitalytics
A blog focused on Twitter culture including links to resources and a running synopsis of popular topics.
http://twitalytics.blogspot.com/

Twitbacks
Lets you create and upload your own Twitter backgrounds
http://www.twitbacks.com

TwitPic
Share photos on Twitter.
http://twitpic.com

TwitStamp
Enables users to place their current Twitter status anywhere in the form of an image. Statuses can be posted anywhere an image can such as blogs, forums, websites, etc.
http://twitstamp.com

Twittad
Provides an opportunity for Twitter users to display advertisements on their user profiles.
http://twittad.com

Twitter Moms
An online community site for moms who use Twitter
http://www.twittermoms.com/

Twitterectory
A directory of Twitter users. Lets users include their profiles and blurbs in a categorized list based on industry and tagged based on the information in their blurb.
http://www.twitterectory.com

Twitterfeed
Offers to tweet the last posts published in a blog via the RSS feed.
http://twitterfeed.com/

TwitterKeys
Provides a floating palette with graphic characters you can insert in Twitter ... just cut and paste.
http://thenextweb.org/TwitterKeys

Twittermap
Provides users with the ability to update their locations on a Google Map and send a tweet with it.
http://twittermap.com/

Twitterment
Search engine for Twitter
http://twitterment.com/

Twittervision
A real-time geographic visualization of posts to Twitter.
http://www.twittervision.com

Twubble
A Twitter application that helps you expand your list of friends by searching your friend's friends to find people who you might like to follow.
http://www.crazybob.org/twubble/

Visual Twitter
Provides the possibility to post images and gives a list of all the last ones sent.
http://visualtwitter.com/