Taxis are ubiquitus in Beijing. They are inexpensive and readily available at almost all hours. Trying to find an empty one during rush hour can be impossible but other than that you usually don't have to wait very long.
When I say they are inexpensive I mean it. The base fare is 10 RMB, less than $2.00 at the current rate of exchange, for three kilometers. After that the meter goes up by 2 RMB (or 35 cents) for every kilometers or five minutes in traffic. A sign in Chinese and English on the windows in the back seat of the cab informs passengers about the cost.
In addition to a ride to your destination, the fare usually also gets you a fabulous language lesson from one of the best Putonghua (Mandarin language) teachers in China - a Beijing cab driver.