GetSatisfaction - Get Satisfaction is the leading customer engagement platform that helps companies build better relationships with their customers and prospects, through the best online customer community.
VisitorEngage - Feedback tab and in-app push notifications for websites based on visitor's behavior on the site.
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
Magento 2 Push Notifications Extension - Want to stay in touch with your customers always?
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.
Socket.io - Realtime application framework (Node.JS server)