(Quick glossary : fic - fanfiction, ie, fiction about the characters written by fans. Slash - homosexual content. No, don't ask, it's very popular.)
Sites
The Lost Land - Susan Cooper Site.
Dark is Rising Fanfic Writer's Guide - details, analysis, background.
Archives
Dark is Rising Slash Archive - does what it says on the tin.
Communities
thedarkisrising - the main community for discussion, fic, etc
dir_slash - The slash fic comm
darkisrising100 - 100-word ficlets