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

Viral Loops - Viral Loops is a viral and referral marketing platform to launch ranking competitions, sweepstakes, pre-launch and referral programs.

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.

Girafi - Get more from your website visitors. Referrals, feedback, e-mails & more.

Zendesk - Zendesk is a beautiful, lightweight help-desk solution.

Referral Candy - Run a fully automated, customizable and easy to integrate referral program for ecommerce.