I was hired by a company to retrofit the popular open source invoice software Bamboo Invoice with a recurring invoice feature. While working on it I noticed the demand for that specific feature was high so I decided to release the mod publicly.
The mod allows the user to set a recurring interval for their invoices. When the interval is reached a duplicate invoice is created with the new date and invoice number. With the PayNow feature you can enter in your Google Merchant ID or PayPal e-mail into the Bamboo Invoice settings page to allow your customers to pay you via these services (a button will be added the PDFs generated).
Check out the Recurring Invoices + Pay Now tag for the latest release.