A stopwatch is a good place to start if you want something readymade and ubiquitous; get a few cheap ones and label them per customer. When I did that kind of

A stopwatch is a good place to start if you want something readymade and ubiquitous; get a few cheap ones and label them per customer. When I did that kind of work though, I had a time tracker application that kept track of which window was in the foreground, based on what project I had active in Eclipse (at the time) I could give a rough guesstimate about what project I spent how much time on.