Personal Activity Monitor tracks time you spend using desktop apps
RescueTime to figure out how I spend my time online. Then it got too complex, and I stopped using it. Personal Activity Monitor is like a vastly dumbed-down version of RescueTime, and I mean that as a compliment. It's free and bare-bones -- all it does is track what applications you're using and for how long.
A big drawback at this point is that it doesn't integrate with Web browsers to help you analyze how you spend your time on the Web. Still, if your work doesn't require constant Web app use, knowing how long you've used a browser overall might be enough to help you manage your time.
This is far from the only application in this space -- alternatives such as Slife and Chrometa are full-featured and impressive -- but PAM is good option for those who want a nice, simple tracker.