I can't completely articulate why I like this but I think it's related to the part of me that enjoys lists - especially the part where I get to see things crossed off at the end of the day.
IOGraphica is free to download. You turn it on in the morning, minimise the little window and forget about it. While you're working, it tracks your mouse movements, showing larger spotches when your mouse has stayed over one place for a prolonged period of time. If you are especially pedantic, you can pause the program when you leave your desk or during lunch breaks - but I tend to let it run. At the end of the day, you have a record of your mouse movements - a beautiful imprint of your productivity.
As Dan says, I'm sure I'll get tired of looking at these, but for now I think they're lovely. Click on any of the images below to enlarge them.
Day 1 - 3.5 Hours
Day 2 - 6.5 Hours
Day 3 - 7.7 Hours
Day 4 - 7.9 Hours