[GeneralMaximus]

I second Toggl. It's simple and gets out of the way. The desktop app even reminds you to track your time if you are actively using the computer for a while but forget to hit the start button.

I charge a weekly rate (for a variety of reasons that I should probably write down as a blog post), so the data is more for my consumption than for my clients'. Still, knowing how many hours of productivity you can sustainably manage per day is valuable for setting your rates and coming up with estimates.