Entry_attribute_counts is a project mainly written in PHP, it's free.
Add custom attributes to <entry> elements in Data Sources
Version: 0.1
Author: Nick Dunn
Build Date: 2010-09-05
Requirements: Symphony 2.2*
*NOTE: you'll need a little change to the core. This should hopefully be present in Symphony 2.2. http://github.com/nickdunn/symphony-2/commit/e6477027cfa7d8be28be54dde0adac3cbdf79be6
This is totally cool but very experimental.