It's really as easy as creating a HIT(? been a while but I think that's what they're called) that says "Beta test this website/app/game" for $0.05. You will probably get a lot of feedback that you didn't expect, even if you don't ask for it.

You can get more sophisticated if you want, but that's all I did. I ended up getting around 4000 seed users for a Facebook app I built in '08 this way.