I was going to recommend BOCP.eu, but it looks like they may just be for Romania. They have a free online invoicing tool that is really easy to use, but being country-specific it probably won’t help you.
Do you need something officially recognized that connects to your country’s Finance Department, or just a simple template that you could update and send to clients? Some countries, like Portugal or Israel, have specific formats that the invoices have to follow. Are you limited by anything like that?