A good friend of mine, Nathan Sandland, has written a time tracking applications for the Mac called ChronosX. He says:
I came up with the idea for the project when I switched from being a PC user to a Mac user last year. The one application on the PC I couldn't find a good replacement for on the Mac was my time tracking app. There are many such apps out there for OS X, but none of them was as convenient to use as the one I had on the PC. This new app solves that problem, and also adds some nifty features, such as direct integration with Apple's iCal calendaring application, so you can see your working time logged right alongside your regular calendars.
I really like that it published an iCalendar feed that you can access as a URL. I don't need to track time for projects right now, but seeing Nathan's program almost makes me wish I did!