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

(Twitter) @ DevX_Com
Follow DevX.com on Twitter.

(Twitter) @ WebReference
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(Twitter) @InternetNews.com
Follow internetnews.com on Twitter.

Blackbird
A Twitter client for BlackBerry smartphones.

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.