Our team loves Slack. We use it daily for all types of communication. So, it was a natural conclusion that our dev team would end up integrating our TeamCity build server into the mix. While we developed this plugin months ago, we just got around to making it available this week. Check it out: TeamCity-Slack by Enliven on Github.
We use TeamCity as our build server along with Octopus Deploy for deployment to various environments. This has turned out to be a fantastic combo - it really provides us a lot of automation and flexibility in our process. The last piece of the puzzle was setting up notifications - there are plenty of things we want to know about - when the build failed, when the build was repaired, or when we've deployed to a QA or production server - but there are also things we don't want to know about, such as when we've deployed successfully to the integration server (which is a lot, since it deploys on commit). Anyway, the bottom line is that this plugin extends TeamCity in such a way that we can easily configure these notifications on a per-project basis.
Caveat: this plugin has been developed for use for our team and it does what we want it to - if you have any issues with it or would like to extend it please keep us posted on GitHub - would be happy to hear from you.