There are a couple in Chinatown, and there is a really decent one in the same building as the Nautilus Plus on St Catherine, near, umm, stanley, I think it is.
These are Japanese style karaoke "clubs" where you go with friends, get a room space, and party with your friends. It isn't a "public" thing.
You can find some public ones, there are mobile karaoke things that go club to club. I think cheers downtown use to do that, maybe on Tuesday or something.
I don't think there are many on the weekends.
MATt