Star Rating System is a free PHP script for adding an Ajax star rating feature to a website.
It uses MySQL as the database & the interface can be styled via CSS.

It has various templates which enables you to create an Ajax or non-Ajax version (that rating can be completed via radio buttons but results are shown in stars).
Several demos for the script can be found here.
Requirements: PHP & MySQL
Compatibility: All Major Browsers
This is an Ajax star rating script that uses cookies, SQL & PHP.
In general star rating scripts have 5 stars & users need to choose one of them. This one has again 5 stars but the selection is percentage based which enables users to select from 1% to 100%.

The website presenting the script, Nomadic Functions, has also some other free scipts that you may want to check, like:
- Color conversion library
- DHTML color picker
- Drag & drop & more..
Requirements: No Requirements
Compatibility: All Major Browsers
Control Suite is a set of 6 high quality widgets and controls built with the Prototype JavaScript framework.
Each script is well tested, highly extensible and unobstrusive.

Suite has these widgets and controls:
- Tab
- Modal
- TextArea
- Select Multiple
- Rating
- Progress Bar
Different approach to star rating:

Suite is very well documented. You can easily reach the details of the APIs and this makes the product much more usable.
Requirements: Prototype JavaScript Library
Compatibility: All Major Browsers
PHP / MySQL Ajax Star Rating Script is an unobtrusive script (which will work if the JavaScript is turned off) which you can easily integrate it with your current content.

This star rating script is not tied to any CMS so you will need some coding skills to integrate it.
It supports configuration of howmany stars will be shown in the front-end.
Requirements: No Requirements
Rabid Ratings is an excellent Mootools based star rating solution which comes with hearts instead of stars by default.
Installing Rabid Ratings is very easy by simply telling the code the database username, password, host address and including a php tag in where we want the ratable item. Rating data is shown in decimals & in heart images Also, the number of votes are displayed too.

Most AJAX ratings widgets use star and half-star images with mouse over events on each star. Rabid Ratings uses a reverse alpha-transparent PNG as a background image to achieve infinitely complex precision (ie, 1.23 stars) and robust visual effects with a very low overhead.
Requirements: PHP 4+, Mootools 1.2
Starbox is a star rating library built on Prototype Javascript framework which you can use to create most types of star rating systems with 1 png image.
In order to add some extra effects to the star rating code, you can add script.aculo.us as well (sample codes can be found in the Starbox website).

Some features of Starbox:
- Re-rating allows you to adjust your previous rating.
- Easily saving your rating with Ajax using the build in callback.
- Styling the Starbox based on status (hover, rated and locked).
- Good seperation of Javascript, CSS and images.
- A ghosting bar that tracks your average.
- Batched building of Starboxes based on cached overlay images.
Requirements: Prototype 1.6+, script.aculo.us for effects.
Compatibility: All Major Browsers