Being a freelancer dealing with different projects I staggered upon the TimeSlotTracker (TST) in my search for a simple yet adequate solution for time keeping.
TST simply works the way I expected, and helps me to collect and document my "slots" for proofing reasons. I love the possibility to define sub-projects with a separate time book-keeping, so a post-mortem analysis is possible where all the time went into just within a project.
Just a small issue that needs to be dealt with, IMHO: The taskbar icon should be different in active and wait mode, so the status of time keeping is visible at first glance.
For me, TST has become a regular for my work days. I can recommend it!