I’m a huge fan of poker, especially Texas Hold ‘em. There are a handful of poker apps for Windows Mobile, but RealDice Texas Hold’em is one that is free, and that works pretty well.
Online poker sites are also developing and rolling out their own Windows Mobile poker apps, which enable players to use the same account they play on PCs and laptops with to play poker from their mobile devices, including game for real money. While the look and feel of the graphics are slightly different and optimized for mobile devices, the basic game play is very much the same, enabling players to enjoy their best poker site from anywhere in the world.
For a broader library of freeware poker games, check out FreewarePPC.