Classroom Deathmatch can be run episodic, but by it's subject matter doesn't promote long term growth of characters (though a Lost style deathmatch could be interesting).
Toon is pretty fun and fast paced, but if you don't want people staring at you for laughing like maniacs, might not be a good choice.
I second All Flesh Must Be Eaten, even though I am not a huge fan of the rolling mechanics, it's fun and allows for some fast paced cinematic stuff.
If you just want to get a game of something in, and don't mind expanding beyond RPGs, the card game Dominion is quite nice, though you might find it difficult to get the first couple games done in 45 minutes.
Avoid GURPS. I love that system to death, but 45 minutes is not enough time.