Slife is a great free app for the Mac that sits in the background and logs how much time you spend in each app that you have running. By telling it when you start a new activity, it will give you an idea of how you spent your time during that activity. So, for example if you are trying to 'Implement a basic Rails app', you can find out how much time you spent reading e-mail, browsing the web or editing your source. I found it quite enlightening and it made me think more about ways that I can stay more focused.
If you want to understand where you time goes and you have a mac, Slife is the app for you.
Thanks to Floyd for the tip.