We've been playing Munchkin as a fill-in for a few years now (Munchkin Fu for preference), but we've had a bit of a change recently.
With a new daughter for one player, serious family health issues for two others, and frequent business travel for some, Munchkin (or cancelled sessions) were getting too common and actual roleplaying too rare. So we're trying out a Mutants & Masterminds game. The system is fast, to get through a complete adventure in an evening, the nature of superhero stories lend themselves well to a very discretely episodic structure and covering for missing PCs, and it's a complete change of pace from the rather lethal and grisly stuff that's happening on the Isle of Dread in the main campaign. Everyone's just going to bring along their PC each week, and if everyone shows up it's D&D, if not, then the superheroes come out to play...