Question2Answer - A Q&A site helps an online community to share knowledge.
DISQUS - Disqus is a global comment system that improves discussion on websites and connects conversations across the web.
OSQA - OSQA is the free, open source Q&A system you've been waiting for.
Answerbase - Add a Q&A system to your website in just minutes, with Answerbase's powerful question and answer software for online communities and customer support.
Askalot - Askalot is a next generation CQA (Community Question and Answer) system.
utterances - A lightweight comments widget built on GitHub issues.