Well, I had been using Quicken Invoice Manager, despite the fact that it locks the data into a proprietary file format, and I even was running Windows 2000 in VMWare to be able to do so, but after setting up a home web server on a Raspberry Pi a few months ago, I not only set up an in-house wiki but also something called Bamboo Invoice (bambooinvoice.org/), which pretty much does everything I need it to do (create about 100 invoices per year and keep track of late payments). After running both Invoice Manager and BambooInvoice in parallel during December, I've moved over to the latter since January 1.

BambooInvoice runs via a Web interface on a machine with PHP and MySQL installed. I've taken precautions to make sure my installation stays private, but that's a lot easier to do when the server is on my side of the router. :^)

Cheers...