Taiga - Project management platform for agile developers & designers
UseResponse - Open-source, self-hosted customer feedback software, live chat and helpdesk system that you can install on your server. Organize documentation using knowledge base and get feedback from social networks with centralized system
TargetProcess - Agile Project Management Web Application
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.
Plan.io - Planio makes web based project management and team collaboration more efficient and fun. It is the perfect platform for your projects, team members and clients.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.