All of these games play excellently as one-shot games. I've run many of these at cons, sometimes as part of my "RPG speed dating" event (Indie by Storm) where I'll run 4-6 games in 4 hours. If I can give you a feel for a game in 40 minutes, then you can have a blast with it in 5 hours!
I have updated this answer with other folks' excellent answers, but added my own understanding of the rules.
I have also annotated each game's entry with a three-plus scoring mechanism in four categories:
- RPG -- is this recognizable as an RPG? compare to D&D to get more pluses.
- 1Shot -- can you play this to a satisfying conclusion in 4-5 hours?
- GM -- does this game require very little GM prep?
- Play -- does this game require very little player prep?
The more pluses, the better! + is okay, ++ is good, and +++ is great!
The Shab-al-Hiri Roach RPG+ 1Shot+++ GM+++ Play+++ (by the creator of Fiasco) is set in a sleepy northeastern college in the early 1900's. You play professors, vying for status. It'd be silly enough, but one of your colleagues unwittingly brought back an ancient Sumerian god in the form of a cockroach. The roach travels from person to person, forcing them to do terrible things that only it can understand. Chaos (and comedy) ensues. Zero prep required. The rules are short and simple. Characters take 15 minutes to make (2 minutes if you know what you're doing).
Carry RPG+ 1Shot+++ GM+ Play+++ is a Vietnam War game, but it's not a tactical fighting game. It's about the hardships soldiers face. You play members of a company of Marines. The characters are pregenerated. The players add Burdens to the stock characters to flesh them out. Takes 15 minutes. Play is serious, often brutal, always rewarding. The rules are fiddly, so the GM will have to know the rulebook.
Montsegur 1244 RPG+ 1Shot+++ GM++ Play+++ is set in a Cathar city in southwestern France in 1244. Catholic crusaders have surrounded the city in a siege. You play some of Montsegur's inhabitants, mostly Cathar religious fanatics. Are you willing to burn for your beliefs? Pregenerated characters, no dice. The game requires some material prep (like printing out and cutting out cards and stuff) but it's a one-time thing that can be done whenever. Fantastic, serious game.
Shock: RPG+ 1Shot+++ GM+++ Play+++ is a game of social science-fiction. It's more Asimov than Piper, if you follow my meaning. Thin details of setting are created in play and developed throughout play. The rules are fantastically simple but produce complex stories.
Misspent Youth RPG+++ 1Shot++ GM+++ Play+++ is set in a dark science-fiction world that you create in play. Setting creation and character creation takes up the first hour or so, if you let people gab and discuss. It's worth it for the buy-in it creates. You play disaffected teenagers who are fighting The Authority. The dice system is simple and can be explained in minutes. The GM's role is mostly narration; no game balance or any of that stuff. Extremely easy to run.
Lady Blackbird RPG+++ 1Shot+++ GM+++ Play+++ is Firefly meets Steampunk. The game is extremely easy to learn. The characters are pregenerated. Lady Blackbird was designed to fill this near-zero-prep niche, in fact.
Fiasco RPG+ 1Shot+++ GM+++ Play+++ lets you play ambitious people with strong motivations and poor impulse control. Essentially, it's the Coen Brothers movie game. Zero prep is necessary and one read-through of the rules will be enough for you to facilitate a game (everyone plays; there is no GM!).
Zombie Cinema RPG+ 1Shot+++ GM+++ Play+++ is light-hearted zombie horror. It's less complex than most family board games and way more fun. It's more toward the "storytelling game" continuum of the role-playing game spectrum, but I think most role-players would see it as an RPG. Everyone plays; no GM.
In a Wicked Age RPG+++ 1Shot+ GM++ Play+++ is hard-hitting swords & sorcery in the vein of the old novels. You pick an Oracle (a set of setting and character tables) and roll -- presto! setting! Within ten minutes, everyone has a character and is playing. The rules are fiddly, so the GM will have to know the rulebook.
Minimalist games RPG+++ 1Shot++ GM+++ Play+++ like 1PG and TWERPS can get you playing quickly. These either have no setting or provide simple supplement books to focus play on a setting. Their simplicity is both a boon and a curse. On the one hand, there are hardly any rules to learn and the GM can just run whatever he or she wants. On the other hand, that puts all the burden on the GM to make the game fun. You're a step or two away from cops & robbers here, but if that's your thing, awesome. I personally think the games I mention above provide just enough structure to focus play and provide much-needed support to the players.