Bangkok Thailand Map
For sheer practicality, get yourself the inexpensive "Bangkok Thailand" map (with red/mauve border and green/yellow background on the cover), which you can find at the airport and in many shops or hotels around Bangkok. It's a no-nonsense street map with bus routes (though it doesn'tyetshow the Skytrain) and also gives you a handy map of Thailand on the back.
Groovy Map Company
This company produces various helpful maps to carry around such as Bangkok by Day and... Bangkok by Night. You can buy these in Bangkok or through their web site.