[xyzzy_plugh]

There's a few. One's Mattermost: https://about.mattermost.com

My company has been using it for a while.

[fairpx]

We'd love to contribute (with design). Are you aware of anyone interested in collaborating. We're looking for something that we can use ourselves and thus contribute to crafting a better experience around the software.

[the_common_man]

Definitely, rocket.chat. It's a proper opensource company and their mobile app could use a lot of love.

[it33]

Hi @fairpx, Mattermost team here, we'd love to have your help!

Here's a page to learn about all the ways the community can contribute: https://about.mattermost.com/contribute/