Jasper Fforde's website is a whole lot of fun to look around. You can see the sites of Swindon, visit the Mammoth Migration Homepage, or windowshop at Pete & Dave's Dodo Emporium. But there are also games and puzzles, a forum, and a guestbook. Even the pop-up ads are "selling" products from the books.
This site is one of the cleverest incorporations of a work fiction into a website I have seen, and certainly more fun than most of those. It is well worth a gander, even if you haven't read the books, though if you do look, you will almost certainly come away wanting to do so.
On a side note, I have read the Eyre Affair and am currently reading Lost in a Good Book, and have really enjoyed them. Most people will find them definitely worth reading.
Books Are: The Eyre Affair, Lost in a Good Book, The Well of Lost Plots.
Other Site Features: Information about Jurisfiction, "the policing agency that works within fiction itself to maintain narrative stability". The Thursday Next Web site. Information in the Writing Section includes signing and tour details, reviews, FAQ, and fun stuff you can download. A form so you can denounce your friends, neighbors, or self to SpecOps. Tons of other stuff.