Valve recently announced on the Team Fortress 2
blog that they are releasing the source files to 10 of the game's maps
, making it much easier for fans to customize them. They also said they'd be releasing more source files for maps and models in the future.
"Here's the list of maps that are included in the update: Lumberyard, Ravine, Badlands, Dustbowl, Granary, Gravelpit, 2Fort, Badwater, Goldrush, and Hydro. For the beginner map makers out there, we hope this gives you an easier entry point by allowing you to alter or edit an existing map, as opposed to being forced to start from scratch. ... We also hope that it'll reduce the amount of confusion around the game logic and entity setup required for the various TF game modes."