Riot.im is out of beta. Riot.im is a free, open source and decentralized chatting platform based on the Matrix protocol for real-time communication offering end-to-end encryption. It can bridge other communications via others protocols such Slack or IRC, and it’s available cross-platform ( including web browsers ). For more info check out the official webpage.
PS: while we at it, here’s why you don’t want to use Slack.
PS2: for everything else there’s Signal.