I like nextbus--has realtime info on the next bus or train that will service any given stop. It does, I think, presuppose that you know how to get from A to B, however. I primarily use it for buses, not trains. For route planning, the wmata site is great, but it gives you scheduled times of arrival, not real time information.

For real time information on trains, http://how....edismetro.com/ (or the more genteel, SFW version, http://howsmetro.com/).