I just released v1.0 of my WordPress Heat Map plugin. This plugin is based on Matt Kingston's excellent Weighted Categories plugin, but with a few additions and changes.
It 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. You can check out a demo of the plugin here.
Hope someone finds it useful.