* Rocket.Chat (https://rocket.chat/)
* Zulip (https://zulip.org/)
* Mattermost (http://www.mattermost.org/)
* Let's Chat (http://sdelements.github.io/lets-chat/)
Oh, and by the way, you could have your own Rocket.Chat instance running in Sandstorm in about 30 seconds: https://sandstorm.io/apps
Update. And I would love to see a modern federated chat protocol to take off. Something like http://matrix.org/.
I don't get the point of complaining about Slack. It works great, it's simple to use and it has an amazing feature set. IRC is pretty terrible. Try doing a drag and drop file transfer. Try using markdown. Try displaying images inline.
I don't get what the problem is with closed source Slack. Do we avoid the phone company because their software is closed source? There comes a point where this FOSS obsession sort of jumps the shark. Worrying about a 'walled garden' for a chat tool is kind of silly. If it bothers people so much, create a FOSS Slack with the ease of use and setup and maybe people would be more interested. But anything that requires me to 'self host' something that amounts to a service means that is a bad use of time. I can pay the Slack guys to maintain chat while my devs work on maintaining our products. IRC is terrible. Try and get someone from your marketing department to use IRC. The neck beard and almost hipster pontification about the 'bloat' of Slack is kind of ridiculous. We have better things to worry about. Slack works on all my devices perfectly. I have a searchable log of everything. I don't have to think or worry about anything. It's simple and works. Until there's a viable alternative that exceeds Slack's ease of use, that's what I recommend people use.
Because you work in an industry were security matters. Such as banking, healthcare, government, government contracting, or any business with more than a couple hundred folks.
Because you provide support and want to customize / integrate that with rest of your data/systems to provide superior service to customers.
I didn't mention IRC in my comment :)
It's interesting how you compare Slack with a phone company. There are important differences between Slack and a phone company. 1. If you give me your phone number I could be talking with you in a few seconds without me even knowing the name of our phone company. 2. You could decide to change your phone service provider and (in some countries) even keep your number.
How would you do these things with Slack? Slack is great, and I like it, but it could stop being great at some point in the future.
(Yes, I realize that Rocket.Chat user couldn't chat with Zulip user, for example. That's why I'm excited about efforts like https://matrix.org/.)
I agree with you: it's expensive to self-host. [1] But when it is possible to self-host, you could hire a specialized company to host the chat system for you. And - if you are careful - you can later switch your chat-system-hosting provider if you want.
[1]: That's what https://sandstorm.io is trying to fix, by the way.
That's an interesting parallel. I suppose it would technically be possible for a company to out-source their internal phone system to an external third-party using VOIP. But I've never seen it done and I don't know how I would persuade a CTO that it's a good idea to make internal communication reliant on the Internet.
Yet companies are falling over themselves to out-source internal IM... why?
Don't know others yet, thanks for the list.
XMPP?
And I would add Facebook to your "until recently" example.
XMPP is an awful soup of incompatible servers and clients, though.
As for XMPP, I think it is still entirely viable as an alternative to using a brand new protocol and brand new clients and servers. But if you want solid off-line messaging, server-side logging and scalable group chat -- there aren't any out-of-the box combination of free/open servers and clients that will work, as far as I've been able to figure out. I assume one would want a solid command line client, solid gui clients for Windows, OSX, Linux/BSD, Android, iOS and Windows phone, along with a good web client.
I don't think you can pick-and-choose a XMPP server and clients that will provide all that, today. It is certainly possible to make that, standing on the shoulders of various open projects -- but the effort would be anything but trivial. I'd be happy to be proved wrong, though!