Taxis are a good way of getting around for short distances and they are cheaper than any other major city in the US. Also, one can share a cab and they’re a good way of avoiding city parking hassles.You pay for taxi hire not by meter but by zone system. You are charged extra for each additional passenger as well as for luggage and for travel on public holidays. Charges vary during the weekday morning and evening rush hours (0700-0930 and 1600-1830). Apart from that, several surcharges apply including ordering one by phone. Tips around 15 % of the fare are expected.  

There are many cabs in DC, Diamond Yellow and Capitol being the two major companies:
Diamond Cab Co.
Tel: (202) 387 6200
Yellow Cab
Tel (202) 544 1212