http://www.jkrowling.com
Doubtless everyone in LJ land will be mentioning this, but it's damn good. And the scavenger hunt aspect of looking for hidden stuff is addictive. So far I've found the very early page of the Philosopher's Stone, the first typed manuscript of the PS, and ancient drawings (page 4).
Edit : And just found the doodle of the Sorting Hat, thanks to