It was asked before, but the most popular were not mentioned in the resulting thread. ;-)
The two most popular are Virtualmin and ISPConfig.
http://www.webmin.com/virtualmin.html
And:
http://www.ispconfig.org/
Virtualmin is the most mature (having been in development for four years, by the author of Webmin), but ISPConfig is also a good light-weight option.
Disclaimer: I am one of the developers of Virtualmin, and Virtualmin is also available in a commercial version (but I've linked to the Open Source version above).