Last November CIO Magazine (Australia) included Freeside as one of the Billing systems to watch. They were right. Since then, we’re even better after integrating a number of CDR formats, adding multi-tenant PBX support, and improving the self service interface and API.  They pointed out that:
The billing functionality includes real-time credit card and e-cheque processing using the popular payment gateways; e-mail, fax, printed and online invoicing; and flexible pricing and rating plans, like anniversary billing and usage based billing. Freeside also integrates Request Tracker, another open source project for support ticketing.
The message must be getting out. The article is available here (opens in new window):