KontrolFreek makes a variety of accessories for XBOX 360 and Playstation 3 controllers designed to make them work better with different kinds of games.
For example, the FPS Freek lengthens the thumbsticks,
By increasing the length of the thumbsticks slightly, precision while aiming is improved dramatically. Your range of motion is increased by 40%, giving you the opportunity to make smaller adjustments and “snap” to targets much faster in first-person shooters. This affords you the luxury of turning up sensitivities to get the closest feel to a mouse possible on a console without an internal controller mod or cheat.
These have gotten pretty good reviews, and for $9.99-$13.99/pair are fairly cheap to give a try.