Time Tracking Tool, a program that I'm evaluating. This is pretty close to what I'd want to use. It has the Play/Pause notion, as well as a task hierarchy. When you activate a task, it shows the time you started, to help remind you if (when) you forget to pause it when you leave the computer. That's probably the chief drawback to the whole problem, when you get distracted from a task (phone call, someone stops by your desk to ask a question), by nature it's hard to remember to tell the computer what's going on.