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

Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.

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.

LiveChat - LiveChat - premium live chat software. Approach customers directly on the website, make connections and drive more sales using LiveChat.

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

Olark - Real-time chat and monitoring of your website visitors through Google Talk and Jabber.