WPChameleon is a WordPress plugin that allows the blog owner to define multiple CSS-based varitations on their blog's theme.
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.
An example of WPChameleon can be found on Linux Loop. Scroll down to the bottom of the sidebar.
Download links and more information can be found at the WordPress plugin directory.