the 'hacked' demo for Carmageddon II was one of the best demos ever. i played that way, way more than the actual full game itself. it also played better on a slower machine, i thought (333 MHz vs 800 MHZ) - the slight slowness made the crashes and running into pedestrians more fun.
one of the neatest-looking things was when you'd hit a broken car piece (a door or bumper or fender or whatever) lying on the road with your car, sending it flying - if you were lucky, it would hit a ped just right and sever off a limb...