I recently wrote up a bunch of split testing code for an application of mine (listy.us) and decided to round out the code, smooth the edges, and make it available under a BSD license. The result is phpScenario (http://www.phpscenario.org), a free object-oriented split testing library for PHP 5.2+. I basically designed the library to be something you could just drop into a project, particularly a Zend Framework project, in a matter of minutes and be good to go. There are posts in the forums that provide very detailed setup instructions. At its very simplest, the library provides one-line A/B testing, and is fairly extensible. I think it'd make a great addition to designer/developer toolboxes.

Right now it's in its early beta stages, but it has plenty of (generated) documentation to help you out. There's also a basic example of its use and results on the main site. You can download it directly if you'd like to try it out.