Driving. But that's a given, right? I know a lot of guys who do this. It's nice to show off what'cha got = )
Though, I'd only risk being seen by a dude- they know what's up, that guys jerk off and aren't that weirded out if it doesn't seem like you're trying to show them. just sort of "oh, whoops, i'm driving and didn't see you in your pickup truck staring at me. oh well, i cant be bothered to stop jerking off." the looks are priceless. occasionally, you get guys who will drive along side for a while and watch you finish. i've only had one guy join in from his truck.
I've never tried that with a woman, just because I've thought they might freak out and think you're a perv.
Other places? Bathrooms, though it's more about needing to get off while at work, and less about it being public. Sometimes in front of a window that I know faces possible interested parties in apartments across the way..
The trick is to never put anyone in an uncomfortable place- it's always their choice to watch. If they do, well... I've gotten thumbs up for blowing a load or two.