One of my favorite new Apple iPhone apps is Meenster (http://www.meenster.com). You can select your Washington, DC Metro Line (Red, Orange, Yellow, Blue, or Green). You then select your station. It will tell you how much time you have before the next train moving in each direction.
If you’re at the pub grabbing a drink, you can check Meenster. If it says the next train isn’t coming for 20 minutes, you have time to grab a quick drink. It’s super convenient.
It’s built with Joe Hewitt’s iUI.