HG, or Head in the Game Controllers, had a brilliant idea to mod Xbox 360 controllers by duplicating some front face buttons onto the back. Why would you need this? So you won't have to move your thumb away from the stick—which usually controls aiming or camera movement—in order to hit the action buttons.
This isn't just for gamers who want a slight edge on the competition, but also for disabled gamers who need to relocate buttons because of an inability to hit certain key combinations. In either case, it's a pretty neat idea that doesn't really cost all that much more ($69 for two buttons on the back or $89 for four) than a standard controller.
360 Fanboy also points out that Gamer Modz has a similar modded controller product that's customized for Halo 3 and Gears of War. They've also got a "turbo" fire controller that will hit the triggers at about 50,000 times a second. [HG Controllers via Xbox 360 Fanboy via Oh Gizmo]