Recs.
Updated
Discord is a free-to-use VoIP and instant messaging app targeted primarily at gaming communities.
SpecsUpdate
Pros
Pro Simple UI
The interface is very easy to follow - you have three sections. On the left are the channels for your server, which you can quickly and easily switch between. The large section in the middle is the main chat area, where each message has the person's username, avatar (or image), and timestamp. On the right is a list of all the people in the chat, where it also ways what games each person is playing (if applicable and set up by the user).
Pro Friendly payment policy
Free account have no tangible restrictions.
Premium account is just a way to say "thank you" to developers. And it work for all servers of discord (not as in slack). And it can be purchased by one who wants. In contrast to Slask, where all active members of the server should be "premium".
Cons
Con Very strict bot restrictions
If you want to migrate to another platform policy will restrict you from being able to use your current Discord account as a platform to communicate the move (or bridge to your new platform). Instead you must create another Discord account, and redirect people there, which you're then allowed to control via script.