I played the original Super Mario Bros. yesterday, after spending 20 minutes ramming the cartridge in my NES again and again. Besides that, most of the games I play regularly at the moment are from the 90's (which is still somewhat old considering the short history of the medium).
As far as I can remember, the oldest game I've ever played on original hardware (so stuff on emulators or the Flash port of Spacewar! don't count) is probably the original version of Pac-man, although I'm not sure the arcade cabinet I played it on was authentic.