How to add a simple "star" flag to posts (a la Gmail).
This works by adding the "Starred" category to the post when the Star button is clicked, and removing it when Unstar is clicked. It was written for a private, single-user site - it doesn't support multiple users having different sets of starred posts.
It uses ajax, so it is fast and doesn't require a page reload.
Tested against WordPress 3.0.3.