Actually.. while I was taking a stroll around WHT (I know.. I'm sorry!), they recommended a panel called BoxBilling. It's actually free and we've been playing around with it and it seems to work great. At the moment we're using WHMCS for our main production as we know it works, but in the future it may be a possibility that we make the switch. No reason at all.. just seems to be something different and seems to be quite unique.
The website is:
Anyways, just thought I'd chime in and show you that.