Please for the love of god just don't use Slack. We have learned absolutely nothing. Let's all jump on the bandwagon of another closed-source, proprietary, walled-garden service and hand over all of our private intra-company communications to a private third-party in another country. GREAT IDEA.
https://plus.google.com/u/0/+JeromeLeclanche/posts/icC6gDToB...
The reason Slack exists and can be so successful yet entirely proprietary is a symptom that IRC is not good enough and that has to be fixed. It is the manifestation of the papercuts we, IRC users, have been having for a long time now. (late edit: If you are interested in fixing this problem, email me, see my profile.)
There are very promising efforts on IRCv3 (http://ircv3.net/) ongoing. I hope they eventually go live to the bigger clients and networks so that we may actually have a good foss alternative to slack (and I don't mean something like Mattermost. Like SirCmpwn said some comments below, having one browser tab per project REALLY SUCKS when you contribute to a lot of projects. I'm in over 20 channels myself, and I do my best to keep that number low...).