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.
This crossover takes TF2 classic Badwater Basin and drops it into Left 4 Dead 2, meaning you can take your four survivors and clear the map fighting zombies, rather than a team of bubbly mercenaries.
While this is a map for Left 4 Dead 2, one of its mechanics from Team Fortress lives on, as the payload can still be moved from one end of the map to the other.
You'd think a mix of the two would be cute, but no. Adding zombies and that L4D music to a Team Fortress 2 map is just...creepy.
You can grab the map at the link below.