A gravitron is the ride at those crappy city to city carnivals that spins real fast. It smells of vomit and sweat and features weight lifting benches on rollers along the wall. Your back is placed upon the bench with you in a semi standing position and the gravitron is activated. Once it reaches speed the benches slide up giving you the feeling of weightlessness. In actuality it is more the centrifugal/centripedal force that allows you to "stick" to the wall.
This guy i work with plans to buy a gravitron and travel the country as a carnie, but we were looking on usedrides.com and they're like $40,000 used!