I made one for Hipchat at a company. I can't remember if it could emulate specific users, or just channels, but both were definitely on my roadmap at the time.

I'm hoping someone can find it so I can bookmark it but I once read a story about a company that let multiple markov chain bots loose in a Slack channel. A few days later production went down because one of them ran a Slack command that deployed or destroyed their infrastructure.