From the "drop dead gorgeous" thread, what cars have you owned, been in or seen that were sleepers?
I had a couple of E class AMGs that were very quick and didn't look like much from the outside. The big V8 in the E63 estate announced itself more than the blown V8 in the E55 saloon, but both were quiet, fairly sedate/boring looking cars that hauled ass.
I took that to the track with some local Benz guys and ran 12.1@115 and the car was stock. Not bad for a two ton sled. At the time, a Corvette Z06 was running low 12s.
My E63 wagon was my favorite because I could haul my dogs. The 6.2L V8 didn't have the grunt of the blown 5.4L in the E55, but it was still a great performer. With 507hp it surprised many a sports car at the stoplight drags
And even though it was low and rear wheel drive, it kicked ass in the snow with some Blizzaks.