Hosting multiple Websites on a dedicated or VPS server means a control panel application to save time on creating sites, e-mails, databases, and so on is of obvious advantage. This is especially the case if it is likely for client sites to be on the server, as control can then be handed over to clients. Here are a few free, open source Linux hosting control panels for easier server management.
EHCP - Built with PHP, this multi-language application's user levels include domain admin, server admin, e-mail user and reseller. Features include script installer, SSL, back-up restore and more.
GNUPanel - This PHP5 built hosting automation application allows easy creation of sites, FTP/ e-mail accounts and databases, as well as end-users if required.
ISPConfig - Supporting admin, end user and reseller levels (complete with individually configurable quotas), this multilingual panel manages Web, e-mail, DNS, database, FTP and stats, as well as enabling setting cron jobs, instant back-ups and more.
OpenPanel - Managing Apache2, AWStats, Bind DNS, Courier-IMAPand Postfix e-mails, PureFTPd and more, OpenPanel offers a slick, simple GUI and a command line interface (optional) for Website control.
Other good panels include:
SysCP BlueOnyx VHCS Kloxo HostinABox
Webmin and Virtualmin are suitable for both Linux and Windows servers, while WebsitePanel is a great choice for Windows servers.