Chex Quest (Chex Quest 3 to be specific)

[Dunkelschwamm]

Yeah, the 3 games are fairly short (5 levels each) but loads of fun. They're more aimed at little kids (which is shown in the difficulty of the games and the fact that your weapons don't kill the Flemoids but instead "teleport" them back to their home dimension where they can do no harm), but the later ones get some difficulty going on (unless you're a hardened Doom fan, that is).