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Server Fault - Server Fault is a Q&A site for system administrators and IT professionals that's free.

Proboards - ProBoards is a host of free forums with customer service.

Reddit - Reddit gives you the best of the internet in one place. Get a constantly updating feed of breaking news, fun stories, pics, memes, and videos just for you.

Disciple - Grow, engage and monetise your community with Disciple community software. Get your own branded community platform on Web, iOS and Android

MetaFilter - Community weblog