Guides

ALTernative OSes List
Sortable list of dozens of OSs, submission Web form, links. Site source code (PHP, MySQL) is free software. [Open Source, GPL]
http://www.cw-portal.com/lele/

Freebyte's Guide to Operating Systems, Free and Non-free
Includes: open source, commercial; Unix (BSDs, Linux); Amiga; BeOS, Zeta; RiscOS; DOS, Windows-emulators; educational; a few related topics.
http://www.freebyte.com/operatingsystems/

Information about Operating Systems
Operating System Documentation Project, OSDP: information on many OSs and their developers; independent objective descriptions, facts, history; in text and screenshots. English, Deutsch, Français.
http://www.operating-system.org/

Jeff Silverman's Operating System Comparison Page
Compares 6 different OSs to try to find the 'best' OS: Windows 98/NT4/2000, Linux Redhat6.2/SPARC/Alpha/Itanium, SunOS PC. Medium size, interesting, useful features table, web server graph. System administrator viewpoint.
http://www.commercialventvac.com/~jeffs/OS_comparison.html

Operating Systems
Brief descriptions, screenshots, links, for OSs: Unix, NeXTStep/Openstep/MacOS X/Darwin, Windows 95/98/ME-NT/2000, VMS.
http://junior.apk.net/~qc/comp/os/

The OS Files
Portal for many x86 PC platform operating systems; details, news, information, secret tips, background, links, availability (current, planned); DOSs, Linux, NetWare, OS/2, Unix, Windows.
http://www.theosfiles.com/

Vintage OS
Information, consulting on DOS, Windows, OS/2, Unix, BSD, Mac, MVS, Palm OS, UNIX, and other vintage operating systems.
http://www.vintageos.org/