WHMCS

WHMCS is billing and automation software for web hosting businesses. It handles client accounts, orders, invoices and payments, and connects to control panels and domain registrars so that a paid order can create a hosting account or register a domain without manual steps. A support ticket system and a client area are included, and modules extend it to servers, licenses and other services.

Many small and mid-sized hosts run it on their own server, with licensing priced by the number of active clients. When you evaluate it, check the modules for the panels and registrars you use, how updates are applied to a customized install, and how payment gateways and tax rules work in the countries you sell to.