Chex Quest, made by Digital Café, is a total conversion of The Ultimate DooM. It was released in boxes of Chex Cereal in 1996, becoming the first game to be included in a cereal box. When it was released, it increased sales by over 200%. It then went on to win both the Golden Reggie Award for Promotional Achievement, and the Silver EFFIE Award for Advertising Effectiveness.
Its a singleplayer wad originally created for the Doomsday engine but instead changed to Zdoom. First of all, no multiplayer wad. It proved a bit too challenging... The wad is a continuation of a old wad (renamed, of course). Fixed all the levels after doom builder stopped being a pest. Fix up the old levels, adding on to them and making them seem more idealistic. And less linear, (but buildings and stuff are linear in real life, right?)