In the annals of self-monitoring tools, here is one that monitors your computer time. It’s a fancy version of time management software. You assign certain tags for various functions and websites — say “surfing” for Digg, Reddit, or Popurls, or “research” for Wikipedia. After you label your activities once, then RescueTime will gather the stats and present you with your accumulative totals in a kind of productivity dashboard. You can get a time budget showing how you actually use time on your computer.
Assuming you can accurately classify which activities are productive, then you can measure your productivity — at least in terms of how much time you spend on “productive” tasks. (This particular software will also require a certain level of trust since your self-monitoring activities are transmitted to the software’s website.) I haven’t used it, though it’s if free and available on Windows or Mac.