Requires PEAR and the Linode API wrapper https://github.com/krmdrms/linode/
Copy linode.php to WHMCSROOT/modules/servers/linode/
Add to WHMCSROOT/configuration.php
$linode_api_key = '1234567890';
Where '1234567890' is your Linode API key
You'll also need a custom field per product called 'Linode ID'