We are currently developing a billing panel, pbobp, intended to eventually replace our WHMCS installation. pbobp is released as free (as in freedom) software under the GNU GPL v3. Currently it's in alpha stage, with it being generally usable but still missing a few key features. In particular it supports ticketing, invoices (with PayPal/CoinBase payments), product definition with groups/configurable fields/currencies (includes basic SolusVM module), shopping cart, automatic service activation/inactivation. Still missing product add-ons, WHMCS importation (username/password import is working; a plugin is needed if you don't want to force password reset, to support the password validation for WHMCS password type), invoice PDF.
The default template is a basic bootstrap layout. A plugin interface is working but needs more callable functions. A few things are defined as plugin interfaces: password storage, service interfaces, payment interfaces, captcha.
It is coded in PHP with modular design, might rewrite parts to some MVC framework. PDO is used for SQL backend.
If you're interested, see https://pbobp.lunanode.com/