While most of these mods tend stay in the first-person perspective, some enterprising souls push the envelope a little further. Team 3's Flipside takes things into traditional side-scroller territory. The game is about an escaped mental patient who can switch between two worlds - one light and sunny, the other dark and horrifying. You can only defeat enemies in the dark world, but you can't take damage in the light one. The keyboard controls might be a ltitle fiddly, but this mod is a must play.

Download Flipside here.