The apf, as it is known, is a long standing project amongst Discworld fans. It's simple aim is to capture every deliberate reference made in the writings of Terry Pratchett and put it up on the web for everyone to see.
Reading it, you get quite a sense of how well researched Mr Pratchett actually is. Computer languages, ancient legends, film noir conventions - they're all in the Discworld series somewhere.
That's not to mention the Really Bad Puns, though.
The afp is divided neatly into books, and each book is linked to the next. Beware! Reading of this site can take a lot longer than you thought...