Sometimes, when you're building an application, simpler is better. At least, that's how Greg Bulmash approached it in his new post about creating a simple rating script for your site with PHP and MySQL.
There are lots of pre-packaged scripts out there to do almost anything you want, but in my experience a lot of them either do too much or don't do enough. Finding that Goldilocks style of "just right" takes a lot of searching and experimenting, or just plain hacking of existing scripts. But for simple functions, writing your own script that does exactly what you want can often be quicker and more satisfying.
He goes through what a rating system is and can be used for before getting to the code (as well as other things that could be tacked on at the end to make it a bit more complex). Then, it's on to the code - first the database layout, then the script to store the rating the visitor gave the item, and the script that shows the averaged results.