A list of dithering resources from around the internet.
A great, comprehensive and technical run-down of many different dithering methods.
The inspiration for Dither it!, this page discusses Low-tech Magazine's decision to use dithered images on the solar powered version of their site.
The quantization library that powers Dither it!
A fork of RgbQuant.js with some additional features. Dither it! might switch to this at some point.
A helpful artcile with great examples an in-depthinfo.
A site very similar to Dither it!
ImageMagick has some robust looking quantization and dithering functionality.
A PHP function for very simple image dithering.
A beefy looking command line interface for dithering images.
A nice little rundown of some error diffusion algorithms, including one developed by the author.