I've used Megabus many times between NYC and Wash. DC.--and paid $1 each way^. The buses are more comfortable than Greyhound, and are double deckers--nicer view of the scenery (if you sit upstairs). They arrived in DC in a much nicer area than GH does.
[About 3 years ago they started to sell trips between NYC and Atlantic City--they sub-contracted with Academy Bus; I was able to get NYC-AC round trip, every week of that summer for $1 and when I got to AC, the casino gave me $25 in cash^]