Well, all the usual places, of course! Lots of places that any "normal" person would fall asleep if they were sleep-deprived, except that I do it even after a good night's sleep. In classes, lectures, conferences, you name it. The subway is a given. Waiting rooms. Hair salon, multiple times. Basically, if I'm sitting down and not mentally active, unless it's first thing in the morning or have very recently taken my meds, I'm a goner.
Definitely planes, trains, and automobiles. Scariest place, I've already posted -- riding on the back of a motorcycle, with no sissy bar. Thank god the driver felt my helmet hit her back as I nodded off, and she pulled over before I could go *splat*. Weirdest? Probably the back seat of an open jeep on safari in the Serengeti. On dirt roads, incredibly bumpy, not enough padding on the seats, incredible scenery, very sunny day -- and I still managed it. That one definitely impressed my traveling companions.
I also fell asleep during my recent ten-year professional recertification exam. That didn't even occur to me as potentially unusual until yesterday. However, after an informal poll of a few close colleagues, I can now say with confidence that this is officially (in my little circle) designated as strange. Who knew?