Update 05/05/14: There’s a new dedicated projects page for the WordPress Heat Map plugin where all further development will proceed. Please refer to this page for information about this plugin in the future.
The WordPress Heat Map plugin provides you with two new template tags for displaying a heat map (aka weighted list) of your categories and your monthly archives. Activity in any given category or month can be indicated by varying font sizes, color intensities or both. There are versions of this plugin for both wordpress v1.5 and v1.2.
You can check out a live demonstration of this plugin at the archives page where it’s used for the “Activity by Month” section.
This plugin was largely inspired by Matt Kingston’s excellent Weighted Categories plugin.