Gaby Vanhegan has made a new BSD-licensed PHP package called Micro-Akismet. He says:
I have written a really compact set of Akismet functions in a single PHP script, to add drop-in scanning and reclassification to any PHP script. It’s under 200 lines long (1/3 of which is documentation) and it’s tiny. I use it on my guestbook and it works a treat!