Linux Tech Networks is proud to help and support the excellent WHMCS billing script. As such, we have made a number of modules and addons for it. Some of these are available for free, and others will eventually be released on an at-cost basis.
Currently, we have 2 modules (addons) available for free, and a third being perfected now

Customized Error Pages
This one's almost a no-brainer, really easy to do. Eventually we'll get something like db logging installed, especially now that WHMCS has gone modular

VCard Download (in production)
This one will allow individuals to download a vcard of a specific client, or of all clients. If all clients are chosen, a dump will be made of the user's client database to a directory of the user's choosing (configurable in the config file, preferrably offline).

Monitor Download(beta conversion, not free)
This module, when released will allow you to configure monitoring on your servers, and give clients access to servers which they can see the statistics.

Sysadmin Module(beta conversion, not free)
This module, when released, will allow you to view server data, allow clients to update server data, as well as handle parsing of email logs for you. All data which could be used to get something valuable (ie: passwords, etc) are encrypted