If you're a biker, the best location for your man cave may well be your garage. There's no reason you can't spruce it up with an orange-and-black color scheme and some sweet flame detailing on the walls. You might not want (or have room for) a huge TV and couch in there, but you could certainly fit a bar and some stools into a corner for when you need a break from tinkering with your hog. If there's a reason why you can't deck out your garage, you can still work the biker look into your man cave in the basement or spare room. Again, orange and black is the way to go, and flames are always a plus -- you may want to stock up on "American Chopper" DVDs while you're at it.
Must-have: It's really best if you have an actual bike on display.