I've been working on updating the Serendipity Karma plugin to include CSS graphical rating bars. (I just love the thought patterns behind "Serendipity Karma".)
It started as a simple code cleanup, to make things more understandable to novice programmers and to comply with Serendipity coding conventions. It wound up nearly a complete rewrite.
I committed it to the Serendipity trunk moments ago; that means it should make it into the next Serendipity 1.3 nightly build. If you don't want to upgrade to the nightly builds, or you just want the plugin on its own, read on...
Update 2008-02-14: Misspelled class name fixed. New zip available.
Update 2008-02-15: Ratings now correctly translated to words. New zip available.