BoxBilling

BoxBilling is a free billing and client management application. A commercial version with expanded functionality is also available. Box Billing was initially released in 2011.

With BoxBilling you can sell any product imaginable - web hosting, software licenses, servers, downloadable products and setup any custom products.

Features:

Supports automated billing, invoicing, product provisioning
Automatically create hosting accounts as soon as the payment is received
Suspend when account becomes overdue, terminate when a specified amount of time passes.
Enable multiple currencies, accept one time and recurring payments
Boxbilling is perfectly created to sell
Shared and reseller hosting accounts
Domains
Software licenses
Downloadable products
Custom recurrent billing products
Helpdesk, knowledge base, news and announcements system.
Intuitive Web 2.0 AJAX powered interface with 100% usability score!