"The Spy Game" by ttfn.tahtahfornow

This is a really lovely AU fanfic; it's subtle Remus/Sirius, but there's so much more to it than that. It's basically written as if the Marauders were muggles and neighbors and went to the same school.

http://www.fanfiction.net/s/3622968/1/The_Spy_Game

The plot to this fic isn't exactly amazing, but ttfn's writing style is really lovely. It's more a collection of viginettes than a fleshed out novel, but the pacing is done well - so well, in fact, that her lack of description seems to be almost the perfect way to write. She never hits you over the head with anything, and when the threads come together, you just have a nice a-ha moment in your head.

If I had any complaints it would be that it's a bit angsty, but not even I, who HATES angst (I really do), was turned off by this fic - the author just hands it all so delicately that scenarios that would be eye-rolling in any other fic just make you desperate for more in this one.