Beijing Map by Lemonita
As Beijing is an everchanging city, I think it's very useful to have a map. Generally, it's quite easy to get around and Beijingers tend to give indications by saying directions (North to that building, East to ... etc.)Also, the map contains all the bus routes that operate in Beijing although it's all squeezed in a part of the map making it hard to read.
Maps usually cost 2 RMB (I bought my first one at 4 RMB from a street vendor!!) and can be bought everywhere, from newspaper stands to people selling them on the streets.