Free / Open Source - Webhosting Control Panel

Webmin
Webmin is a web-based interface for system administration for Unix. Using any modern web browser, you can setup user accounts, Apache, DNS, file sharing and much more.

ISPConfig - Hosting Control Panel Software (Open Source)
ISPConfig 3
* Manage multiple servers from one controlpanel
* Web server management
* Mail server management (with virtual mail users)
* DNS server management (BIND and MyDNS)
* Administrator, reseller and client login
* Configuration mirroring and clusters
* Open Source software (BSD license)

ISPConfig 3 : Debian 5.0, Ubuntu 8.10 – 10.04, CentOS 5.2 – 5.5, Fedora 10 and 12-13, OpenSuSE 11.1 – 11.2
ISPConfig 2 : Mandrake Linux 8.1 – 10.2, Mandriva 2006 – 2010.0, Red Hat Linux 7.3 to 9.0, Fedora Core 1 – 6, Fedora 7 – 13, SuSE Linux 7.2 to 11.2, Debian 3.0 – 5.0, Ubuntu 5.04 – 10.04, CentOS 4.1 – 5.5

Ajenti Server Admin Panel
An admin's tool for a more civilized age, providing you with a fast and secure way to manage a remote Linux box at any time using everyday tools like a web terminal, text editor, file manager and others. 

Vesta Control Panel - Simple & Clever Hosting Control Panel

froxlor - Server Management Panel

CentOS Web Panel - Free Linux Web Hosting Control Panel

VHCS - Virtual Hosting Control System
hosting control panel for simplified webserver management

VHCS is a open source web-based interface for easy web server administration for Linux. VHCS is designed for hosting and internet service providers and IT professionals.

VHCS manage Apache vhosts, DNS, email and FTP accounts, quotas, create statistics and much more.

Account Services Manager
Account Services Manager is a web-based account manager for UNIX web hosts so that clients can manage services such as FTP accounts, POP3 accounts, Subdomains, Password-protected folders, etc. without having to email support.

AlternC - Trac
Opensource Hosting control panel. AlternC is a set of user-friendly mass hosting management software. It is easy to install and based on open-source software only.

Easy Hosting Control Panel

Ehcp is a Hosting Control Panel (hcp).
hcp is a tool in a server to facilate the process of hosting domains, emails, adding domains, ftp users and so on.

supports Ubuntu 6-10.04, debian 4,5, all debian based linux'es

WebsitePanel

WebsitePanel is an easy to use control panel for Windows hosting. It allows you to manage multiple servers, it has a robust, scalable and secure architecture.

With WebsitePanel you can easily manage all your web sites, FTP accounts, databases and other resources from a single place.

Windows

Sentora - The open-source web hosting control panel!