Timing (http://timingapp.com/) does a nice job. It also allows you to define "projects" which are applications, folders, or websites which then get tracked together. Useful for client work or in figuring out how you actually spend your time on a spike, for instance.
Thanks, I started using it, it's already 5 hours. I see now it doesn't track very well. Then I've read in their website that it doesn't support firefox. It does track websites with firefox but has some errors.