This is the PDF invoice generator used by Return to the Source, provided here for everyone to use under the AGPL license as part of our free and open source philosophy.
Invoices.created.today is a web application to create beautiful invoices quickly. It is primarily useful for independent contractors (freelancers), but it's also suited for small to medium businesses. It is just an invoice generator, not an accounting system, although it could be easily used by one.
There are tons of invoice generators out there, but one very similar online tool is Invoice Generator by the company Invoiced. The main technical difference is that Invoices.Created.Today operates server-side as opposed to client-side, meaning you can log in from any computer and have access to all your invoices.
While the source code is free as in beer and speech, the invoices it produces belong solely to the user that generated them, of course.