Froxlor has Debian packages available and a .tar.gz for production installations. Only Debian is officially supported, but it seems possible, with a little effort, to install it on Ubuntu as well. Froxlor is licensed under GPL 2.0, with source code on GitHub.
Froxlor is an open-source web control panel with a very clean interface. There is IPv6 support, a ticketing system, and an integrated reseller-customer messaging system. It is ideal for ISPs and similar organizations.