OS-List - Loosely categorized, unalphabetized list of 100 OSs. - http://www.uw-pc.com/os.html
Open Source Ring - Alphabetized list of many links, OSs, projects. - http://www.webring.com/hub?ring=os
Paul Hsieh OS Page - Loosely categorized list of about 100 links to developer resources for creating OSs, emulators, alternative OSs. - http://www.azillionmonkeys.com/qed/os.html
Microcomputer Operating Systems - Some 70 OSs that run on microcomputers, PCs. User interfaces in earlier systems are command-line, character-oriented; later systems are mostly GUI-based. List, with brief descriptions, no links. From West Virginia University. - http://www.csee.wvu.edu/~jdm/classes/cs258/OScat/micros.html
A Catalog of Operating Systems - Classified, alphabetic listing of most operating systems, past and present, distributed commercially or reported in English-language literature, platform oriented. From Computer Science 258, West Virginia University. - http://www.csee.wvu.edu/~jdm/classes/cs258/OScat
Alternative OSs on the Net - Huge, comprehensive, alphabetized list of operating systems and OS link sites. All links, no comments. - http://www.d1.dion.ne.jp/~r_high/GeekOS/indexos.html