Talkus - Chat with your website users from Slack
sup3rS3cretMes5age - A safe, simple tool for one-time (self-destructing) messages
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.
BURN.link - Send secure, encrypted, self-destructing messages
Intercom - Intercom is a customer relationship management and messaging tool for web businesses. Build relationships with users to create loyal customers.
Boop - A scriptable scratchpad for developers. Free & open source!