D.C. Metro, Washington D.C.
Most of DC attracions are aroun metro stops.
There is not timetable.
You might to have to wait for a train for more than 10 min. at night and Sunday.
The DC Metro system is very efficient and clean. The maps are very clear (I did't get lost even once - a miracle)
The Washington Metro is GREAT! It's cheap, runs late in the night, and will get you almost anywhere you want to go.