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.
BoxBilling - Complete client management, billing & support software.
Blesta - Blesta is professional client management, billing, and support software.
InvoicePlane - Free and open source invoicing application to host on your own server.
Work 365 Apps - Technology that scales with you. Use Work 365 so you don't have to.