Valve releases TF2 map source files

Valve's released source files for some of the official Team Fortress 2 maps, as detailed in this blog post.

Maps included are Lumberyard, Ravine, Badlands, Dustbowl, Granary, Gravelpit, 2Fort, Badwater, Goldrush, and Hydro.

For aspiring map-makers, this essentially allows you to edit existing levels instead of having to start from scratch.

Full thing through there.