Beeminder is an excellent commitment device which you can use, amongst other things, for tracking the number of minutes you spend on a project.
There is a useful bookmarklet to help you fill in data for a Beeminder graph that tracks minutes. Click on the bookmarklet while you are looking at your Beeminder goal page and are about to start work on the goal. The bookmarklet will record elapsed minutes in the goal's data field for as long as the goal page is open. When you have finished work, all you need to do is click the submit button and the correct value will be submitted.
