This time I'm looking for a concept, not a specific fic.
The concept I'm looking for is any and all crossover fics where a misplaced individual is launched into a universe they know everything or close to everything about and they use their knowledge to affect the course of the series. The two fics I've read so far that use this concept are:
The Wild Horse Thesis
Gosunkugi sends Ranma into Evangelion to get rid of him, but NGE is Ranma's favorite show, and he uses what knowledge he has to befriend Rei, Misato, Kaji and Asuka and try to stop Gendo's plans for Third Impact. One of my favorites.
The Nighthawk Chronicles
Xander from Buffy the Vampire Slayer winds up in a Teen Titans/DC comics universe after an accident with a 'powerful magical doohicky' and some C4. He takes on the identity of Nighthawk, gets Slade's attention, and gets in a four-way romance with Raven, Starfire, and Jinx, among other things. Another really good fic.
Anyone got more to add?