Chex Quest was a DooM total conversion (TC) developed by Digital Café, and released as freeware by Ralston Purina on boxes of Chex Cereal as a promotion back in 1996.  Chex Quest consisted of a five level episode in which the player got to take on the role of the Chex Warrior and rid the peaceful planet of Bazoik (Yes.  Bazoik.) of the evil mucus based life forms, the flemoids, which have invaded from another dimension.  Upon completion of the game, users were told to go to chexquest.com, where they were able to download a second episode, Chex Quest 2, in the form of a patch wad. (Note: chexquest.com still exists, but no longer has anything to do with Chex Quest.)

The Chex^®Quest CD-ROM and its decorative cover sleeve.

Chex Quest was touted as a "non-violent" game, despite the fact it was a (very slightly) modified version of DooM.  Supposedly, none of the flemoids were ever actually killed, but were simply teleported back to their home dimension via the "zorch" energy fired from the player's weapons.

A third five level episode, Chex Quest 3, was created by fans of the game, using levels and graphics from other DooM mods.  It can still be found on various web sites around the net, but is generally not considered a true Chex Quest game, because of its stolen content.

At present, a mod for the Eternity engine, Chex Quest III: The Flem Wars, not to be confused with the above mentioned Chex Quest 3, is currently in the works.  This mod is intended as a parody of the game Contra III using the characters from Chex Quest.

Another new mod, this time for the new DooM 3 engine, called Chex Trek, is currently in its early stages.