Open Source Operating Systems & GUIs
Pardon the dust - page under rebuild

Operating Systems - PC platform

ReactOS - Windows Open Source clone!

Flavours of Linux/UNIX

FreeBSD UNIX	Red Hat Linux	SuSE Linux	Ubuntu Linux	Debian Linux
Mandriva Linux	Gentoo Linux	Arch Linux	Slackware Linux	CentOS Linux (Enterprise Server)
		Fedora Linux

 
Knoppix Linux – useful as a test OS – as it loads it does the best job of identifying problem hardware which might life difficult when you install a full distribution later. Here’s a ISO bittorrent download sitefor Knoppix and another

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Distros
For a full summary (and confuse yourself with choice), check
Wikipedia
 DistroWatch

Ubuntu Linux (UNIX) is the cleanest to install and provided with the best additional packages. Install with the GUI, FF/TB & Open Office.

GUIs - PC platform
GNOME GUI
KDE GUI
 
More useful stuff on UNIX, etc.
Linux.org
Linux on Notebooks

Embedded platforms - under construction
Embedded Linux on new devices

Mobile platforms - under construction
Phones
Symbian - Symbian Foundation
Android
LiMo Foundation
openmoko

Tablets
Nokia N800-N810 - Nokia - Nokia dev - Tableteer -  Maemo