Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
Live Chat by Sumo.com - Instantly communicate with customers, prospects & visitors
Zopim - Live chat is a faster and more personal way for you to engage your customers. Free for 14 days. No credit card required.
Crisp Chat - Multi-channel customer support software
GoSquared Live Chat - Beautiful, simple, and free live chat for your website
Mutiny - Personalize your website for each visitor