Plesk Billing (formerly ModernBill) is also very popular, and I see this set to increase significantly since purchased and re-developed by Parallels as it will be the best option for hosts using Plesk by a long way due to tight integration.
On that basis if you make a module for WHMCS there's also a significant market with Plesk Billing - especially since it doesn't already have a built-in system (like WHMCS does) so Parallels would presumably be more interested in this as there's something in it to make their product more popular instead of just one-sided deal (like the WHMCS<->Cerberus option is in many respects).
I think this is probably one of the most important features to have added as a plugin.
The two largest helpdesk competitors to Cerberus (Kayako/Vision) have this integration already and the WHMCS integration is the biggest thing stopping us from purchasing this product and leaving Kayako.
I'll have to 2nd (or 4th) everyone on this. I loved Cerberus back in the v3 days, and it seems even nicer now. I think integration with one of THE leading hosting billing platforms out there would definitely be a good thing...
Not an official plugin, but I did make a start at a Cerb4 support module for WHMCS.
http://www.cerb4.com/forums/showthread.php?t=1674
Hey guys, we also have another new request for AWBS support. So if you have any interest check that one out too.
CHD-1258
We may have something soon for anyone watching this..
http://forum.whmcs.com/showthread.php?t=38694
Matt
New Feature
Open
[Plugins] Official WHMCS module