The only game systems I've had time for have had rich depth but fairly simple rules (QAGS, James Ernest games, etc.). As a gamer on the go, what do you play for quick but complex and enjoyable games?

I happen to have a Palm Treo 680 on which I have a game called PalmIshido. The object of the game is to place tiles, and the tiles must match two ways: in color and in image on the tile, so placing tiles takes a strategic mind. You want to place as many tiles as you can. If you have a Palm, it's a game worth trying out. Also, I tried out a game called Senet, an ancient Egyptian game, believed to be the basis upon which backgammon was developed. Senet was really fun. Also, I have a Palm game called GoMoku. The object is to place five stones in a row while you keep your opponent from doing the same. Simple rules, not-so-easy to accomplish. The actual game is called Go. I believe it's a Japanese version of chess. It is played to exercise one's ability to plan strategically. I love it.