We use Slack almost exclusively. We love it, and all of our students are on it. The alumni channel is great because it keeps all the people that went to our school connected to our community.
We started with Wave, but they were not developing new features fast enough, which is understandable, because as a free tool it probably does not have the resources. Recently we switched to Xero. We did some research before adopting it and it seems to be the best tool. We are very happy with it.