Kloxo/HyperVM might be worth a look - open source/free and released under AGPL-3.0 license.
I can't say whether or not development is keeping pace with bugs at this point - you should know, if you use this software, that there were some issues with security (which turned into some huge headaches for hosting providers) a couple years back.
Edit: Though the same can be said for Webmin/Virtualmin - whichever platform you choose, you will definitely want to subscribe to the project's development mailing lists and keep an eye out for vulnerabilities (malicious users love it when they can compromise a hosting platform, so take precautions).