WHMCS - WHMCS is an all-in-one client management, billing & support system for online businesses.
HostBill - HostBill is a powerful and flexible, all-in-one automation, billing, client management and support platform for online businesses.
WeFact Hosting - WeFact Hosting is an easy-to-use and beautiful designed billing solution specially made for hosting...
Blesta - Blesta is professional client management, billing, and support software.
InvoicePlane - Free and open source invoicing application to host on your own server.
BoxBilling - Complete client management, billing & support software.