Valve today released an update to their Source SDK, and included within were the source files for most of Team Fortress 2's original maps. Translation? It's now a lot easier to edit Team Fortress 2 maps.
Rather than forcing you to build your own map from scratch, which can be tricky, this update now lets anyone with the time and patience to "alter or edit an existing map" instead. The map files included in the update are Lumberyard, Ravine, Badlands, Dustbowl, Granary, Gravelpit, 2Fort, Badwater, Goldrush, and Hydro.
The best part? This is just the beginning, Valve saying "Our plan is to get more of our TF content source out in the SDK, so look for more maps & model sources in the future".
Get 'em, boyos! [tf2]