Time Tracker v2.
Time Tracker’s Website offers an interface with which you register, login and do you all your time tracking. You simply create a task, start the timer and then stop the timer when that task is complete. Think of it as a slightly sophisticated online stop watch.
Simply tracking individual tasks as a stopwatch event is okay, but for the time data to be put to use you need more. That is why Time Tracker provides a simple inline overlay that reports how long you spend on tasks.
There is also an option for exporting to XML, Excel (CSV) and OPML formats. The exported data can then be manipulated for other purposes you might have for examining time usage (or for timesheet billing). This is a purely online service so the drawback is that you have to manually log in online and stop/start your task timer. It is a basic tool, but if it meets your needs, it is a good choice and is totally free.