Amanzimtoti on the WordPress testers’ list suggested a core change that would insert the class “current-cat” into the link for a category, if someone’s browsing the archives under that category. It would allow WordPress users to style their themes so as to highlight the relevant category–a good idea. (Amanzimtoti invited everyone to check it out on his blog.)
I figured there must be way to do it using a plugin, so I came up with the Filosofo Highlight Current Category plugin. It should work with WordPress 1.5 and newer. Just extract one of the files below, upload, and activate.
Note: The features in this plugin were added to WordPress back in version 2.0. You do not need to install this plugin in a recent version of WordPress.