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!
Akismet 
I might have to give this a try on my site, as Akismet was causing all kinds of problems following the minimalist uprade from WordPress 2.0.6 to 2.0.7