My man cave is the basement... I have my 55" tv down there with every gaming system imaginable with two large couches in the main area, behind there is my little "office" corner of the basement where the computer sits along with all of my audio recording gear, and in the room next to that is the music room, where I have all of my guitars, a keyboard, my amps, PA and my drumset. The other little room doubles as a utility/storage/workout room, where I have the punching bag and a few weights. I love it: it's dark and kind of dingy (like most basements are) and it's a cluttered mess, which means my wife rarely goes down there. I would love to put some money into it and hang some drywall and lay down some carpet (the wood paneling and concrete flooring is oddly pleasant to me, but it could be SO much more cozy...)
I also have the 4 car garage that also serves as a good man cave, but since it's unfinished, there is no insulation or source of heat, so up here in Minnesota, it doesn't make for a very pleasant place to hang out 8 months out of the year... One day though, if I ever run into some extra cash flow, that will be fixed.