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AskBot - Askbot enables users to create Q&A forums.
Ask Me Help Desk - Ask Me Help Desk is a question-and-answer site.
Blurtit - Blurtit is a Q&A website. You can ask a question or answer on any question.
OSQA - OSQA is the free, open source Q&A system you've been waiting for.
AllAnswered - Your Team's Collective Brain All-In-One Knowledge Management System Wiki + Q&A + Workflow + AI