"In this edition of 5 Open Source Products to Watch, we take a look at billing systems. That’s right, there are open source applications available for invoicing and billing customers. They’re open source, Web-based and can be extended and integrated to suit specific needs.
Best of all, they don’t cost a red cent.
"1. AgileBill
AgileBill was released as a commercial product in 2004 and then open sourced by its creator Tony Landis in 2008. AgileBill is a billing and invoicing application suitable for the membership/subscription type of business model, including Web hosting companies, ISPs and VoIP providers.
AgileBill features plug-ins for payment processing, provisioning, and interfacing with third-party applications and services. It has also given rise to the AgileVoice and AgileISP VoIP and ISP billing applications, respectively."