AWBS!

BILLmanager is a linux-based control panel for automating client management, invoicing and billing. This software specifically developed for hosting service providers will help you manage and take complete control of various aspects of your business:

WHMCS is an all-in-one client management, billing & support system for online businesses. WHMCS handles everything from signup to termination, with automated billing, provisioning & management. With WHMCS, you're in control with a very powerful business automation tool.