For the privacy conscious, there's also https://www.secure.chat/ https://www.secure.chat/ if you want a slack alternative with end-to-end client-side encryption

For the privacy conscious, there's also https://www.secure.chat/ https://www.secure.chat/ if you want a slack alternative with end-to-end client-side encryption. You can choose which channels or even individual messages you want private and encrypted. Of course, the tradeoff with end-to-end encryption like that is the content is opaque to the server and can't be indexed or searchable, but you get to choose how you want to make that tradeoff when you send the message. Disclaimer: I worked on secure.chat in the past.