Chinatown buses have been around for over 15 years now -- they serve the East Coast and Southern California to San Francisco (and do a leg to Las Vegas, too). With curbside stops and not a lotta amenities, they're a super cheapie option for seaboard hops. Learn more:
Megabus rolled from the UK to the US in spring, 2006, tried and left the left coast, and is now firmly established in 13 Midwestern-Northeast states and Ontario, Canada, around a Chicago hub. Seats can go for as little as a buck on the limited luggage, stationless cruisers. Learn more:
BoltBus is a budget alternative partially parented by Greyhound and serving the Northeast US. Like its rivals, Boltbus offers little in the way of luxuries, but does have wifi (wow). Learn more:
Greyhound buses still rule the road in the US, with far more routes and potential flexibility for you than any of the mega cheap buses. And it's not all that expensive if you apply a student discount. Look into it right alongside the cheap buses-- you may be pleasantly surprised.