HelpScout - Help Scout is a simple, straightforward way to provide excellent support
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
HelpNinja - Simple & Affordable help desk
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
SupportYourApp - Connecting talented people with the blooming IT industry to help them grow faster by outsourcing support.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.