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
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
HelpNinja - Simple & Affordable help desk
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
SupportYourApp - Connecting talented people with the blooming IT industry to help them grow faster by outsourcing support.