UserVoice - UserVoice integrates easy-to-use feedback, helpdesk, and knowledge base management tools in one platform that empowers users to speak and companies to understand.
Stack Overflow - Community-based Q&A part of the Stack Exchange platform.
Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community
The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Askalot - Askalot is a next generation CQA (Community Question and Answer) system.