Let's see...I did it on the floor once...is that WILD and CRAZY enough for ya?!?!
All kidding aside...I have fond memories of a bridge on the University of North Carolina at Charlotte campus...
...in a few different cars...but after reading this thread, it seems to be a staple...
...My favorite experience was by far on top of a picnic table in a local park...I feel like a pervert for doing it in a place where kids play...but we cleaned up our mess and didn't leave anything innappropiate or questionable for anyone else to find later.