Recs.
Updated
Slack is a team communication app that can allows team members to discuss, chat and share items in different groups, with integration with more than 100+ 3rd parties integration such as Hangouts, Soundcloud, Dropbox etc.
SpecsUpdate
Pros
Pro Channels and private groups
Channels are groups inside the team chat that can allow team members to discuss anything related to the channel topic that a team member is using. The channel can be set either to public chat or set to specific chat allowing only some team members. Any topic that is not related to the team can be posted into the #random channel, a channel that is created automatically.
Cons
Con Expensive when you need to upgrade
At $6.67 per user / month (or $8 if billed monthly) , Slack is significantly more expensive than the competition if you need features such as unlimited integrations (more than 10) or unlimited message storage (more than 10,000). However, the free version of Slack includes unlimited users.
However if you need only unlimited messages you can use storage services like Slarck to upload then browse and search your entire message history, while staying in Slack's free plan. So with a combo of Slack+Slarck this con is not that major.
Con Hidden max limit of free users per channel
Slack says that their free accounts support an unlimited number of users, which is true. However they don't mention that there is an undisclosed maximum number of users per channel (8462). For a large open source community, this is something to keep in mind.
Con No self-hosting available
If you are worried about third-parties getting access to your data you should consider self-hosting. With self-hosting you are in control over where your data is stored, who has access to it. You will also not be vulnerable to exploits of a third-party provider.