I have been using
Webmin (and
Virtualmin) pretty regularly for the last couple of years. I really got tired of some of these slow admin control panels you see for servers these days. Cpanel, Plesk, etc., are fine control panels but I find them bloated, slow, and resource hogs. Webmin is extremely lightweight, and also free. Not only can you admin your sites, but you can pretty much admin the entire server using the webmin GUI alone.
The Virtualmin add-on helps with multiple sites and has a very functional free version, along with a commercial one with more bells and whistles.
One of the first things you notice when you first try Webmin out is the speed. Everything runs faster.
I just cannot say enough about this software and what it has done for my workload.