TaskTime is a great application that helps you keep track of time spent on tasks and a whole lot more.
TaskTime3 can make tracking time on projects a snap. Generate custom invoices then print, email, or save them. Unlimited clients, unlimited projects for each client.
I've been using it for a few days now (thanks for Matt for the tip!) and while I'm not currently using some of the advanced features like invoicing, I've found it to be a great way to keep track of time spent on a project.
If you're looking for a good, lightweight timekeeping application, this is it.
It's shareware, free to try, for Mac OS X only.