Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
Talkus - Chat with your website users from Slack
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.
Crisp Chat - Multi-channel customer support software
Smallchat Beta - Slack integration for chatting with visitors to your website
HubSpot Conversations - Live chat, bots & team email -- free.