I recently stumbled across an excellent Firefox Plugin called MeeTimer, created by Andy Mitchell.
MeeTimer records how long you spend on each site whist surfing the web and allows you to view a list of most visited sites per day and overall. It’s easy to categorise sites as “work”, “communication” and “procrastination”, or create your own custom categories. The idea is that by understanding your web usage habits, it should become easier to change these to become more productive and ultimately have more free time!
The nag screens are also an effective way of saving time by monitoring your online activities and giving you hell in the form of a percentage of working week wasted… Great work!

