Toggl is an online system for creating and tracking your timesheets. It's great if you work for a company that bills its clients based on how long you spend working for them. Just a couple of clicks each time you start and end a particular task, or type of task, will allow you to produce reports that show what you've been doing and which client should be paying for it.
Even if you don't work in such an agency-like environment, or if you don't work at all, it's a great way to find out just what you do all day! Create the necessary task types and projects, make a note each time you start or finish a particular type of work, and then view the reports.
There's even a selection of phone apps and browser plug-ins, to make the recording process really quick and convenient.
You'll find details and the sign-up page at www.toggl.com. The system is commercial and has a monthly fee, but a basic account for up to 5 users is completely free forever.