I started my freelancing career with billable.me. Moved on to complexer tools by now, but I definitely made a few thousand euro with the help of your free tool.
I'm using InvoiceFox (https://www.invoicefox.com/) these days. It's really cool because it can remember all the different clients I have so I can just click "Invoice Company X for Y" and it also does some analytics of earnings and payments discipline.