Coming very soon are two bus services that will offer very cheap fares between Philadelphia and New York. Fares that are as low as 1 dollars (or even free at first).
The first one that will come in will be for the Greyhound/Peter Pan Boltbus service. This service has just started in New York, and will soon stop in Philadelphia on April 10, 2008. These buses have fares as low as a dollar (although initially the promotional fares have been free), and will feature amenities such as leather seating. You may be able to ride this bus to DC as well, as it currently runs an NY to DC route. For more details, visit www.boltbus.com.
The second one is a UK based service called Megabus. It already operates popular service in the U.S. in the Midwest, and will operate its eastern division out of Elizabeth, NJ on May 30, 2008. This will run an inexpensive route from 30th Street and 5th and Market, Philadelphia, to Penn Station, New York. The fares will range from 1 dollar to 14 dollars one way. For more details, visit http://www.megabus.com/us/.