I don't know about full fledged CRM software, but here's a list of Billing Applications compatible to cPanel and WHM:
Commercial Applications (Listed Alphabetically)
* Account Lab Plus - Netenberg.com/accountlabplus.php
* Advanced Webhost Billing System (AWBS) - AWBS.com (Formerly: DRAMS)
* ClientExec - ClientExec.com
* HostAdmin - DreamCost.com/?page=hostadmin (Requires purchase of add-on module to work with cPanel/WHM)
* ModernBill - ModernBill.com
* PhpHostBot - idevspot.com/PhpHostBot.php
* Platypus - resellers.TuCows.com/backoffice
* Ubersmith - Ubersmith.com
* WHM AutoPilot - WHMAutoPilot.com
* WHM Complete Solution - WHMCS.com
* WHN Billing Professional - WebHostingBilling.com/whnbilling.html
Free Applications (Listed Alphabetically)
* Auto Host Manager (AHM) - AutoHostManager.com
* phpCOIN - phpCOIN.com (Requires add-on module to work with cPanel/WHM)
* Solid-State - Solid-State.org (Requires add-on module to work with cPanel/WHM)
As always, if anyone knows of anything not on this list, just give a hollar and I'll make sure it appears on similar future posts.