The creator of this side-scrolling interpretation of Team Fortress 2 intended to enter it in a map-modding contest last year but missed the deadline.
Steaky may have missed the cut (here's another top-flight entry in the contest) but he says he "plan(s) to finish and release a full version in the future." A quick Google search reveals the guy's built two maps already, (plus this one). I've no idea how far along the project is or how it would be implemented, but it is sure fun to look at, and think about what could be. Go for it, man.
Thanks to Gøran T. for spotting this and sending it along.