One of the advantages of both Team Fortress 2 and Left 4 Dead 2 sharing an engine is that it’s relatively easy to share content between the two. Like, say putting zombies in a Team Fortress 2 map.

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, ...