The best April Fools' gags are things you can play—little alt-reality vignettes from a silly universe with a broad possibility space. For instance, what if Id didn't just make seminal FPS Quake, but made a version of Quake that was actually Superhot? It'd look a bit like SUPERQOT; a free, downloadable, real thing that you can play right now.
As in Superhot, time only moves when you do. The difference is that SUPERQOT is the shareware version of Quake, modded and packaged with the DarkPlaces source port. It functions reasonably well for a time-based version of the classic shooter, and even has multiplayer support.
To download SUPERQOT, go here.
Update: A reader and CoD modder, BraXi, took notice of SUPERQOT and has created his own mod: SUPERCOD! Check out the video. (And thanks for the e-mail, BraXi!)