Backgammon board.

You might not lose your shirt, but you might lose interest in online poker. (From The Minneapolis Star Tribune)
Overrated
Like so many others, I tried playing online poker, and while I didn't lose my shirt, I did lose interest. Texas Hold 'Em and its kin are best when you can look an opponent in the eyes or, at least, the tinted sunglasses. It's not nearly as much fun revealing a full house to an anonymous cartoon character.
Underrated
During a recent vacation, a friend reintroduced me to backgammon -- and proceeded to crush me 23 times a row. In preparation for our rematch, I've been playing online backgammon at Spin32 and become thoroughly addicted. The free game allows you to compete on any of 10 levels. I'm only on level 2, which means I'm not quite ready for face-to-face combat. Yet.