Ravatars will generate and assign random icons to the visitors leaving comments at your site. It can optionally show Gravatars as well.
A random selection of Ravatars using the default source images. When choosing your own images, you may want to pick only images of certian colors that match your theme. Or you may want to use only nature shots if you have a blog about hiking. Also take a look at my site grok-code.com for an example of Ravatars that can be created from a different set of source images.