PHP Thumb is a light-weight image manipulation library aimed at thumbnail generation. It features the ability to resize by width, height, and percentage, create custom crops, or square crops from the center, and rotate the image. You can also easily add custom functionality to the library through plugins. It also features the ability to perform multiple manipulations per instance (also known as chaining), without the need to save and re-initialize the class with every manipulation.
More information and documentation is available at the project's homepage: http://phpthumb.gxdlabs.com