Talkus - Chat with your website users from Slack
Smallchat Beta - Slack integration for chatting with visitors to your website
Chatlio - When you have a business to run, your average instant messaging software just will not do. You need an internal presence upon which your entire team can collaborate on. Read more about Chatlio.
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
Zopim - Live chat is a faster and more personal way for you to engage your customers. Free for 14 days. No credit card required.
Avochato - Text messaging for your business.