Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
Olark - Real-time chat and monitoring of your website visitors through Google Talk and Jabber.
tawk.to - tawk.to is a free live chat app that lets you monitor and chat with visitors on your website or from a free customizable page
LiveChat - LiveChat - premium live chat software. Approach customers directly on the website, make connections and drive more sales using LiveChat.
JivoChat - JivoChat is a live chat that enables you to talk to visitors on your website
Crisp Chat - Multi-channel customer support software