Webmin certainly does NOT need OS X 'Server'!

Actually webmin is very much an interface to control most of the same services as OS X server does.

You can make an ordinary OS X do much of the same that a OS X Server does, and in that case Webmin is a great tool.

You can also use Webmin together with OS X Server, to do more advanced control of services than OS X Servers simplistic 'Grandma' interfaces allow - or in cases where OS X Servers GUI simply fail to work properly with the underlying open-source services.

Webmins interfaces can appear very befuddling at times, and many times you also need to Google documentation and fiddle directly with config-files, but overall it's a help - and a serious product - for free.