There are a lot of good systems out there. Seriously, we have enough gaming systems doing enough different things that you could probably find a game for anything you want to do. Looking for high school teen drama? There is a game for that. Looking for sex magic witches? There is a game for that. Looking for giant robots where one player can play the AI and the other the pilot for the mech? There is a game for that too, and before you marvel at the variance I just listed there I'd like to point out that you can find a game for all those all from one designer too.
Still, there is one kind of game that while it isn't what most of us are looking for all the time is still fun, especially around the holidays. Games you can get into quickly. Games designed to address your needs for a night, or weekend, of gaming but without wanting a ton of thought being put into characters because you're not going to be playing them for the long haul.
In short, one shots.
So, aside from some of the small games I referenced from the above designer, what are some of your favorite one shot systems? What makes them tick? What makes them fun?