WHMCS - WHMCS is an all-in-one client management, billing & support system for online businesses.
FusionInvoice - Self hosted invoicing for freelancers and small businesses
HostBill - HostBill is a powerful and flexible, all-in-one automation, billing, client management and support platform for online businesses.
Invoice Ninja - Invoice Ninja is a free, open-source solution for invoicing and billing customers.
Blesta - Blesta is professional client management, billing, and support software.
Bahaquote - Bahaquote offers complete online invoicing and billing software designed to seamlessly integrate with any business.