There aren't any dedicated Chaos websites out there that are really worth it. There is EldarOnline for us Chaos players (such a shame!).
There are lots of "This is my chaos website" kind of deals, where some joe-shmoe puts up the same description of their patron god (usually stolen from a 2nd edition codex), a few poor quality pictures of their miniatures and maybe an army list or batrep. Nothing cohesive and major, however.
I know of a few Chaos bulletin boards, but most of them are very quiet these days. The B&C has an okay Chaos section. I highly recommend the 40k-Chaos yahoogroup.