The RechnungZentrale V2 is a software which can create and manage bills and smalerparts of them, I call the billingpoints.
The software is completely webbased and can manage unlimited admins(persons who can create bills and billingpoints) and customers of the admins. The softwe is quite simple, so don't expect to mutch goodies.
This is an invoice program for freelancers or small businesses. The groovy feature is the email notification process. You can set the program so that clients are automatically sent a thank-you email when an invoice is paid. And you can also send them notification emails when their invoice is online. Clients can view/print invoices when they need, and they can view their invoice history. An administrator uploads the invoices into the database through a web admin system, and can edit invoices and client details.