On the Blue Parabola blog Keith Casey asks the question "how do you measure an individual's contribution?"
In the past few weeks, I've been working fast and furious at getting web2project to our v1.0 milestone. As part of that effort, I track open issues, problematic modules, community feedback via the forums, death threats via all methods, and other related aspects.
He suggests a few different ideas for measuring how much a user has contributed: lines of code, commit count, issues they've reported, number of issues closed or community involvement. Of course, none of these can truly measure how much an individual has participated in a project, especially since it could be a mix of several of them combined into a whole as the "involvement persona" of any given person.