WHMCS - WHMCS is an all-in-one client management, billing & support system for online businesses.
Kajabi - Kajabi is the only Knowledge Commerce platform today with everything you need to market, sell, and deliver your knowledge online.
BoxBilling - Complete client management, billing & support software.
Tendenci - Tendenci - The Open Source AMS is a cloud-based all-in-one management SaaS solution for Non-profits and Associations.
HostBill - HostBill is a powerful and flexible, all-in-one automation, billing, client management and support platform for online businesses.
YourMembership - YourMembership provides membership management software integrated with social networking features.