I am currently obsessed with Marauders era fanfiction, mostly because it alleviates my sadness about their eventual fates. (It's not a spoiler, we all know that one of them is already dead by the first book :3) I hang out on several different HP fanfiction sites, and these are the best of my finds so far:
http://unknowableroom.org/44/1 Silly Love Songs, primarily a Lily/James fic but involves all the Marauders
http://unknowableroom.org/228/1 Twice As Bright, a one-shot from Petunia's POV about the first time Lily brings James home
http://unknowableroom.org/479/1 Habit, a one-shot about how James is always following Lily around on dates (x3 just read it, it's good)
http://www.timeturner.net/fics/index.php This whole SITE is dedicated to Marauders era fanfiction, so go nuts!
http://www.thepetulantpoetess.com/viewstory.php?sid=3348 The Ultimate Trust: oneshot, Lily and James' wedding
http://fanfiction.mugglenet.com/viewstory.php?sid=70321 A Heart To Heart In History of Magic: James and Sirius pass notes in History of Magic
http://fanfiction.mugglenet.com/viewstory.php?sid=68711&chapter=1 The Marauders, basically the story about the Marauders' time at Hogwarts from around the fifth year
http://www.firewhiskey.net/story/296/1108/ Stupid Evans - James loses a bet and faces the prospect of being Lily's house elf for a month.
The Harry Potter FanFiction Guild
