WHMCS Bridge plugin by zingiri integrates your WHMCS support and billing software into WordPress providing a seamless and consistent user experience to your customers.
There is also WHMCS Bridge Pro (aka SSO) a professional extension to the WHMCS Bridge plugin that takes the integration between WHMCS and WordPress a step further, current cost: $19.95 USD. Details: http://www.zingiri.net/plugins-and-addons/whmcs-bridge-sso/
Thanks to the theme inheritance feature, you don’t need to style your WHMCS installation anymore, the integration ensures that your WHMCS installation looks and feels like your WordPress site.
Thanks to the single sign-on feature (WHMCS Bridge Pro plugin), your customers can sign in once on your site and comment on your blog postings, share information with their peers, order hosting plans and pay their bills.
See it working at: http://www.zingiri.net/portal/