Addon Module Developer Docs
Addon modules allow you to create both admin pages and hooks to extend WHMCS further.
Modules can consist of just an admin page, or just hooks, or both. And they are all managed through the Setup > Addon Modules interface.
Once activated, the modules will display in a dedicated "Addons" dropdown menu within the admin area for quick & easy access from any page.
Management options consist of activating and deactivating of the modules, aswell as access control which allows full admins to define exactly which of the administrator roles are allowed to access each addon module.
We have a Video Tutorial demonstrating addon module management & access control @ http://www.whmcs.com/tutorials.php?i=addonmodules
Documentation
Documentation providing everything you need to know to create an addon module can be downloaded @ http://www.whmcs.com/members/dl.php?type=d&id=63
Example
We have created an example module file which you can use as a basis for all your custom modules to help with getting started, and that can be found at the path /modules/addons/addonexample/ within your WHMCS installation.