So, you are looking for:
the best free time tracking software for an freelancer
While I love open source and free software, there are still some types of software that I am quite willing to pay for.
I consider what I risk losing or gaining! If I'm billing a client at $80/hour and I can more accurately and easily track my time with a piece of software that costs $50 to $100 then all i have to to do is track 2 extra billable hours and the software has paid for itself.
This software I've been using for years to track my time and it's paid for itself many, many times over. It's especially designed for consultants and it's easy to either time myself while I work or add time records after I've finished.
Responsive Time Logger is the software. Their website is: www-ResponsiveSoftware-com
It's free, of course, to test out their software. I've called tech support a few times and they are very helpful. Yes - they are based in the USA.