Send quick PRIVATE messages to a group of Twitter users. Not really what I'm looking for StartPad - a community of twitter's sharing relevant tweets (stuff about Seattle Startup community, meetup support, etc)

Quoted: Send private Twitter messages to specific groups of friends.

Quoted: a valuation of something like $90 per unique monthly visitor is hard to justify on pure traffic
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one thing, though, that is true of Yahoo! and not of its rivals--it gets social apps
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daily use of [Yahoo] Groups rivals MySpace and Facebook
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If Yahoo and Microsoft together can figure out how to make money with Yahoo's unique social platforms, they will have created some sustainable advantage. If not, well, $44 billion is a lot of money to pay for mere traffic.

This Google Group seems to contain a hierarchy of discussion groups to organize the topics in this group. I'm not sure how they set this up like this, but it's pretty cool for a high volume user group (like this one for Google Page Creator).

Quoted: Free forums for everyone! Start a free forum on Nabble; add a forum to your website, customize a forum skin to exactly match your website; provide free message board hosting; create free discussion group; archive mailing list; free customizable cobrand forum to match your website.

GAP is a system for computational discrete algebra, with particular emphasis on Computational Group Theory. GAP provides a programming language, a library of thousands of functions implementing algebraic algorithms written in the GAP language as well as large data libraries of algebraic objects.