Valve has released an update to Team Fortress 2’s [Source Engine's] SDK that will include the source files for many of the official maps. The map files included are: Lumberyard, Ravine, Badlands, Dustbowl, Granary, Gravelpit, 2Fort, Badwater, Goldrush, and Hydro.

Valve hopes this will inspire even more map making, by giving people a jumping off point, rather than starting a map from scratch with a whole new idea. They have also said that they plan to do more things like this in the future with the SDK.