[Update 2014-01-27]
The system grew from being an "emulator" as I called it at first, to be an actual port of Moebooru to PHP. If you're looking for the files, you can find them in Github here.
Before I decided to work on this in 2009, I don't think I made a deep search on a Danbooru-like page for PHP.
(Defining variables here)
Anyway, I'm not even sure how to catalog Danbooru (just in case, there're tons of 18+ pics there), but it's an image board that works with tags, you can download and use its source code to make your own Danbooru. Sadly, it's made on Ruby and works with PostgreSQL I think? Not Windows-user friendly.
Then we have Imouto, based on Danbooru but modded for good, cooler than 'booru I'd say.
