When I'm in Philadelphia, I often spend weekends with friends in Washington, DC. While I would prefer to fly or take Amtrak, bus travel is so inexpensive that I usually opt for that.
Up until a few months ago, my only bus option was New Century Travel, a Chinatown bus service that offered r/t tickets from Philadelphia's to Chinatown to Washington's Gallery Place/Chinatown for $15 o/w or $28 r/t. Although the price was right, legroom was extremely tight, the overhead reading lights rarely worked, and there were no electrical outlets.
Enter Megabus, now offering service between Philadelphia and Washington. At $13 o/w ($26 r/t + a $0.50 booking fee [but even cheaper if you buy in advance]), not only is the price more reasonable, but legroom is better, each seat has an electrical outlet, and WiFi is free. I type this as we are rolling past Baltimore.
Busses get a bad rap in America. Please don't rule them out, especially Megabus, if you are traveling in the Northeast corridor or along any other routes they serve.
Next stop: Park Hyatt Washington.