I have 200 Gig of "Other"

Before downloading Mt Lion, I had more than 3/4 of my HD available. Now I have near to nothing because I have 200 GIG of Other. What the **** is it and how do I deal with it? The computer is running fine, but I suspect in a while it won't be. Surely there shold be a way to identify this stuff and get rid of it.

I tried Disk Utility and repairing permissions, but it didn't help.