I just wanted to discuss a few addons that are out there that I Have found to be extremely useful. I will list a few of them. They are not free, but they are worth the expense to have them.
My personal top 5 favorite ModulesGarden addons and what they do. (Some descriptions taken off their website.)
1) SolusVM Extended VPS For WHMCS - This gem allows you to automate the provisioning of virtual servers to your clients and offer SolusVM products inside WHMCS. Xen, KVM and OpenVZ! Adds functionality that it'd take months to code on your own. A+++++++++
2) SolusVM Extended Cloud For WHMCS - This is similar to VPS only allows customers to provision multiple instances in a cloud-like fashion. This is amazing as well, since with the next addon, you can do hourly billing!!! A+++++++++
3) Advanced Billing for WHMCS - Allows you to dynamically bill your clients for actual server resource usage like disk space, bandwidth or CPU usage. A+++++++++
4) cPanel Extended For WHMCS automates accounts provisioning and allows your clients to manage their web hosting accounts in the WHMCS client area! The module eliminates the necessity of logging in to cPanel as it relocates many of its functionalities and advantages directly to your WHMCS. This is an awesome module that is extremely useful. A+++++++++
5) IP Manager For WHMCS allows you to easily add and manage IP subnets as well as automatically assign IP addresses to your servers, products, addons and configurable options. Along with it, you will gain the possibility to freely assign subnets to your clients, who will be afterwards enabled to order additional IP addresses from them in the client area. All these without even leaving your website! A+++++++++
Other addons I highly recommend:
1) Lara Admin theme - This is a replacement admin theme-- why should the customers only get the pretty interface? Maye WHMCS pleasing to your eyes as well!!!
2) Advanced server status - Allows you to add server stats in basically a sever monitoring fashion. Shows which services are online or not. Very useful!
I use all of the above modules (and more) and find that it helps my business tremendously. Some of the modules are harder to configure and setup than others, but ModulesGarden has some pretty extensive setup guides. Plus, they offer some sort of support-- maybe not the best, but they appear to offer it.
I hope this has been helpful.