I’ve used Megabus a few times and the service was very good and inexpensive.
I paid around $12.00 roundtrip for my wife and I to go from NY-DC and a drop more for us to go from NY-Baltimore just to mention a couple of times we’ve used them.

Megabus is giving away 200,000 free seats from January 4- March 1, 2012. They travel to over 70 US cities & Toronto. Their buses offer free wifi and power outlets so you can use your travel time surfing the web. When I used the wifi it was spotty at times but that was a couple of years ago. Maybe they have improved the service since.

To book free tickets, use code GOFREE at the Megabus site.