Check the tech and modifications board for tons of info..

So far:
I've done LED taillights in the back (brake, turn, reverse), and also changed the front marker light to white LED to go with my xenon lowbeams and fogs.

Black tint all around except for windshield.

Painted lower front grill silver to match the rest of the trim.

Cupholder mod

Also I wouldn't really call it a mod, but in the 9 months I've owned my xfire, twice I've pulled off all 4 wheels and waxed the entire rim inside and out. This makes the wheels stay a lot cleaner than most crossfires, especially the inside of the wheel, which otherwise gets extremely black. So it kind of gives my wheels that 3rd dimension that most crossfires lose after a few thousand miles. Even if you keep the outer rim sparkling clean it's hard to get the inner that way unless you actually pull off the wheel. This is something every new crossfire owner should do, if you care about that kind of thing. Some guys have done the no brake dust pads and have had success.