JitBit Helpdesk - JitBit is a web-based help desk and ticketing software solution
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
osTicket - osTicket is a widely-used open source support ticket system.
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
HelpScout - Help Scout is a simple, straightforward way to provide excellent support
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.