Mattermost is a Slack-like tool that we use for chatting and organising.
There are browser, desktop (Electron), mobile, and terminal clients available, as well as tools to 'bridge' between other chat software like IRC, Slack, Discord, so you can access it from (hopefully) wherever best suits you.
To join you need an invite link: https://chat.florencesoc.org/signup_user_complete/?id=2a7237f68937b2c4a99ca25c156e6915
Programs for accessing Mattermost. You can also just use your web browser!
There a a bunch of programs that are designed to collect-together browser-based applications. For example you can axxess your Mastodon, Mattermost, Slack, and webmail all together in one Electron app instead of having to run a whole bunch.
Bridges and Plugins
Clients which can be used for Mattermost with the help of other software.