Right now you can go onto Live, unless you work or something, and pick up Wolfenstein 3D by iD software.
This is the classic shooter that really started the first person shooter genre. Sorry Doom. It's also one of the only games I can think of that allows you to shoot Hitler. Repeatedly. Seriously, how many bullets can one man take? As far as I can tell, it has the original sprites and levels intact- should be some easy achievements in there somewhere. And the game is really fun too, don't want to forget to mention that.