Unfortunately a fair percentage of RGB fic sites seemed to be hosted on AOL Hometown, and were therefore nuked a few months ago when the site shut down. I almost cried, thinking I'd lost some of my favourites. Thank goodness for the Wayback Machine, but there are still some good sites out there if you dig around hard enough.
...aaaaaaaaand Adultfanfiction.net, also with a low signal-to-noise ratio, and it's pr0n, which makes it that much more frightening.
...though I do have to admit a ridiculous amount of love for "Oh Hell-y Night" which gets a little shaky in parts, and features some wildly OOC moments, but I still love it and the punchline is totally worth it.
Tobin's Apocryphon (archive site of RGBAdultFic list), has a lot of my favourites, such as:
"The Name Game", which I somehow missed on my first few browses through the site. Gives me a huge case of the giggles every time.
"The Shower" I have two minor quibbles: exposition should not take the form of the main character talking to himself, because it eventually gets creepy, and Egon would never misuse the past subjunctive tense like that!
My particular favourite is "Host," despite the initial urge to run like hell as soon as the underlying trope became obvious (having spent time in the Harry Potter fandom, I developed an aversion to it), but it's an interesting variation on the theme, and funny enough to put me at ease.
Karen's RGB Fanfiction, home of "I Get a Kick Out of You," a trilogy based around yet another idea that normally sends me running in the other direction, but is also a very peculiar kind of bittersweet like a knife to the heart, but so well-written that I buy it. Apparently I can accept just about anything if it's written well enough.
"Play Us a Song, Dead Piano Man" has a great title and gives me the frigging creeps because I hate dummies, mannequins, old dolls, etc. One too many ventriloquist dummy horror shorts on USA when I was a kid, I guess.
Fic by Jeffrey S. Morris--I'm glad to see this has a new home, but alas "Perfect World" doesn't seem to work at the moment.