If you’re one of those people who only now discover GNU/Linux for gaming (with Steam and all), you might find this useful:
http://www.ryochan7.com/projects/antimicro/
I’ve tried several tools that map controller input to keyboard keys, but AntiMicro is the most reliable and easiest to use in my opinion. Many GNU/Linux developers forget to implement native Linux joystick device support into their games, but often this is crucial for games like Stealth Bastard Deluxe or Super Meat Boy. If you encounter such a game, AntiMicro will let you play it just fine with any USB controller, ranging from cheap-ass $2 Chinese crap up to expensive Logitech controllers.