Update: Ivymedia has all the cheap East Coast buses you could ever need.
Those of you who’ve come searching for the cheap DC-NY Chinatown bus may also want to check out Hunt Valley Motor Coach in Baltimore. It’s not quite as cheap as the Chinatown bus, and DC’ers will have to drive or take the train up to Hunt Valley (which is so far North from Baltimore it’s practically Canada), but at about $35 round-trip it’s still a pretty good deal, as compared to about $80 round-trip on Greyhound. (Thanks to Wayne for the tip.)