Matrix is the protocol, Riot is a Slack-like implementation of the protocol: https://riot.im/
You can use the default (free) Matrix homeserver with Riot and you'll basically have a free chat system without any setup (besides installing the client).
I've been using Riot for a while now, and while Slack does seem a bit more polished, having client-side encryption for all my messages gives Riot the upper hand: I don't worry about sending passwords or sensitive info as much as I do with Slack.
I thought it was kind of expensive at first but then I saw the price of slack. ~$1.50/user/month seems very fair when you see what slack and email cost per user
You can use the default (free) Matrix homeserver with Riot and you'll basically have a free chat system without any setup (besides installing the client).
I've been using Riot for a while now, and while Slack does seem a bit more polished, having client-side encryption for all my messages gives Riot the upper hand: I don't worry about sending passwords or sensitive info as much as I do with Slack.