LOVE the horse racing game! I would definitely play that, whether against the computer or against real players.
Any of those old fair ground games would be cool with Bitcoins riding on the outcome
Aside from 'classical' betting games, I would like to be able to bet on the outcome of an online boardgame like The Settlers of Catan, Risk or Carcasonne against friends or strangers. With some percentage for the house, of course. Try to enable people to have fun by playing, not just by the potential gain and they will return.
