I’ve recently released a PHP library for Akismet, the comment spam detection service. There’s already a plugin available for WordPress, but the integration is so tight that you can’t easily use it in other PHP projects, and the existing library linked to from the website apparently doesn’t work well with HTTP/1.1.
The library is free to use and available under the MIT licence. I will be using it on at least one of my PHP sites, so you can rest assured that any bugs caused by a change in the API will be spotted and fixed quickly.