Slack

Slack is a team based communication platform that allows for open communication and collaboration between team members.

It can be used for one-to-one messaging, but is mostly used for open team communication, using public and private channels that relevant team members join to collaborate with their colleagues.

Great for asynchronous communication (where not everyone needs to be online at the same time).

There is a free plan that can be used, with very generous limits on users, messages, etc.

https://slack.com/