See "Social Media Tips for Online Marketing Plus 30 Social Media Sites" in the Did You Know...? section of Webopedia.

(Twitter) @ DevX_Com
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(Twitter) @ WebReference
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(Twitter) @InternetNews.com
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BIGGER.twitter
We offer you the ability to post messages to twitter which are much longer than the limit of 140 characters.

Twitter
Twitter is a service for friends, family, and cob .workers to communicate and stay connected through the exchange of quick, frequent answers to one simple question: What are you doing?

Twittermaps
Provides users with the ability to update their locations on a Google Map and send a tweet with it.

Twitturly
Whether people use, TinyURL, Snipurl (snurl), or link directly to the final URL in their tweets, we always count the "votes" correctly because our spiders actually visit every single site before it gets displayed here.