phpCOIN
phpCOIN is a free billing and client management application. phpCOIN was initially released in 2002 and today is used by over 35,000 people worldwide.
phpCOIN (Clients, Order, Invoices, and Notes) is an e-commerce/online-store solution that includes a variety of modules (store setup and display, customer management, invoicing, payment, and support) within a single interface.
WHMCS
InvoiceNin...
Simple Inv...
ClientExec
Blesta
BoxBilling