Poker. Either strip or for chips.
I used to play backgammon for money but haven't played in years.
Guilt edit:
I completely forgot about these:
The summer I turned 18, I was working in Manhattan and my boss' son and I would pool and place OTB bets together during our lunchbreak, based on whether or not I like the horse's name.
When I moved down to Florida, I had a group of friends that were Jai Alai fanatics. I spent probably about six months watching the hotties play and betting on the ones I thought were the hottest.