HelpScout - Help Scout is a simple, straightforward way to provide excellent support

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

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

Service - Customer service issues solved for you, on demand, for free.

Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community

LiveMessage from Salesforce - Make mobile messaging a cornerstone of customer service