Developer: Digital Café
Release Date: 1996
Source Code Release Date: December 23, 1997
Legendary for being the first video game released in a cereal box (or at least the first good one), Chex Quest is a fantastic FPS game using the DOOM engine but eschewing the violence for cartoon zapping. Essentially a kid-friendly DOOM total conversion with new levels, this marketing tool has continued to entertain, especially if you have a kid who wants to play a retro first person shooter.
The first game in the series was released on a CD-ROM in boxes of Chex, the second game was available for download on Chex's website, and the third game was commissioned in 2008 (!). There's a reason why everyone from AVGN to John Oliver have commented on this game.
The WAD files are available for free here.