Come on, I don't want to sound generic, like i'm just choosing my favourite author, but JK Rowling's site is amazing.
It's almost like a game. When it first came out I spent hours finding all the clues so that I could get the "bonus content". Just the little things, like if you type in "magic" or 62442 (the same as the code to get into the Ministry of Magic) you get bonus stuff. It's so interesting to see JK Rowling's original notes, in her own hand writing and everything.
And the extra content is amazing. All the little things that you really wanted to know and stuff you didn't even suspect... like Dean Thomas' past.
I loved that whenever there was a "big announcement" like the release date of Deathly Hallows, she'd take the "do not disturb" sign off and I would literally spend HOURS trying to figure out how to get the clue. I think I got it once. The rest of the time I ended up cheating and looking at the clues on mugglenet because i was so desperate to see what the announcement was. It really made things fun
And then there's the other things she does... like The Wombats. That would of taken her days to write all those questions with all that original content. They were so much fun.
I love the layout, like its her own desk. And it's all so accurate. She chews gum like there's no tomorrow because that's what she does now instead of smoking. So the desk is littered in gum wrappers. And I loved that we could vote on the questions we wanted her to answer.
Basically I just adore her site. I could spend hours on it. Fun fun.