Wine works reasonably well for a lot of applications. I've even played StarCraft on Linux through Wine (though I couldn't get the sound or online play to work quite right) so it works okay even for simpler games. As far as higher end games though, I don't think Wine is quite adequate. I've heard of some other Windows libraries for Linux that are more geared for games and DirectX applications, but they weren't free and I can't remember their names.