Unlike Mercedes-Benz which produces AMG badged versions of just about every model in its lineup, BMW has maintained greater exclusivity with the M badge. Until last year's launch of the X6 M and X5 M, there have only been four M models (not counting the M1), but one M got as far as prototyping and testing, but never produced: The M8.
Powered by a 550 horsepower version of BMW's original M70 V12 engine, the M8 was meant to challenge the Ferrari 456 GT. However, following testing of the single prototype, BMW canceled the program and built the 375 hp 850CSi instead. The M8 prototype was long thought to have been crushed, but it turns out that it was stashed away in a corner and turned up in the BMW museum in Munich. Check out the BMW Blog for more shots.
[Source: BMW Blog]