As a Mac user, I'm a huge fan of On The Job (http://stuntsoftware.com/OnTheJob). You enter in a task and the charge per hour, hit play and go to work. You hit stop when you're done and it builds the invoice for you automatically.
Seconded. For some reason I'm always on the look out for something better to come along, but On the Job is the one app in this category I actually use.
That sounds perfect. Although they offer a download and also have an option to pay seemingly with no explanation whether the free version is crippled somehow? I'm not currently near a Mac so I can't check what the difference is between them.
I think all of Stunt Software's products come with 20 day trials. My experience is with version 2 of On The Job but the newest edition looks even better so I'm probably going to upgrade shortly.