Recently, I was researching for a potential billing system for one of our clients. The requirement is to be able to create rate plans that has certain fixed monthly cost and overage charges based on usage. Also, they prefer open source. Here are some of the tools that I reviewed and my comments on them.
A cgi script based billing system. It seems like they are pushing it as a hardware & software solution. The user interface seems to be okay. It seems to have good functionality. There is an online demo site http://www.freeside.biz/freeside/demo.html
A free open source billing system. With enterprise features and a robust architecture, jbilling is the choice for businesses of all sizes. Built on the Java 2 Enterprise Edition to ensure security and scalability. This seems to support bundling and packaging of items and allows complex pricing rules.
CitrusDB is a web based customer care and billing software solution (CC&B) that uses PHP and a database backend (MySQL) to keep track of customer information, operational CRM, services, products, billing invoices and credit cards, and support information. The feature set is very minimal. You can create basic plans, add customers, create invoice.
opentaps Open Source ERP + CRM
opentaps is a complete open source platform for your enterprise based on Java. Its sophisticated features and modern architecture includes a full ERP and CRM solution plus mobile connectivity and built-in business intelligence tools. It is more than a illing system and it might be a overkill if you are looking for just a billing system.
A Plus Bill (Not an open source)
A Plus Bill is a PHP/MySql based online invoicing, billing and client management system with online help desk and auto billing capability via cron jobs(in linux) and Scheduled Tasks (in windows). It includes a user-friendly backend to keep track of clients and repeated charges. It is created for web hosting companies, ISP providers, general invoicing, and customer management. It supports the basic features.