WPChameleon allows you to offer your users multiple variations on your website's theme using CSS.
After launching a new theme, I found that my readers couldn't agree on what colors they find easy to read. WPChameleon lets you define custom bits of CSS to define "variations" on your current theme. Your users can then select which variation they wish to use.
Users' preferences are stored in cookies, so no login is required.
Note: Version 1.3 fixed a bug relating to cookie paths that could, in some cases, cause the plugin not as intended. Please make sure you update.
If you decide to use this plugin on your website or blog, please email me a link. (See FAQ for my email.)