There's only a handful because the following sites will be more than adequate to the task of providing links. It's just to get one started. Perchance the web ring will be helpful...? Try here for some odds-n-ends of links.
Premier fanfiction site. A ton of links beside.
Mary Jean Holmes writes some of the most incredible RGB fic you will ever lay eyes on. The Holly and the Dryad is a personal favorite.
The Real Ghostbusters: Out of Control
Tory, aka Icewoman's, extremely funny website!
Jeff Morris' HUGE archive of fic crossing a lot of fandoms. Quality RGB fic to be had here.
Tam's Real Ghostbuster Fanfiction
Tam's fic and a lot of other people's beside. Good stuff.
ERobin's page, with awesome RGB fic s as well as some other stuff.
Archived by Labidolemur. Contains all the fanfic, poems, and some art posted to the RGBFanfic mailing list.
A site for fan discussion, plus tons of other interesting things.
Ghostheads United
For goodness' sake, go look at what Dr. Venkman snuck into the Spengler Files! I assume it's still around somewhere...
A big, thriving messageboard!
Kinda closed down sorta...but I guess it's an archive now.
A site where you can upload your own fiction and have it seen by thousands. (Which means some is good and some not so good!)
Follow that Blip!