CPM

California Digital, Inc.
A company that still sells CP/M.
http://www.cadigital.com

Commercial CP/M Software Archive
A collection of CP/M software, including source codes and documentations.
http://www.retroarchive.org/cpm/

Commodore and CP/M
Technical description of Commodore's implementation of CP/M Plus in C128, including a couple of tools.
http://www.herne.com/cpm.htm

CP/M 3 patches for Spectrum and Amstrad
CP/M patches for Spectrum +3 and Amstrad PcW16.
http://www.cpmspectre.pwp.blueyonder.co.uk/cpm/cpm-indx.htm

CP/M - The First PC Operating System
Information about the history of CP/M and Digital Research. Links to various CP/M resources.
http://www.digitalresearch.biz/CPM.HTM

CP/M Center
Information about CP/M and links to various CP/M resources.
http://www.gaby.de/ecpm.htm

CP/M Main Page
Resource on technical internals, software, utilities, many links; by John Elliott.
http://www.seasip.demon.co.uk/Cpm/

Joe Wein's DRI page
Some notes about Gary Kildall, CP/M and Microsoft from a former DRI programmer.
http://www.joewein.de/dri.html

OAK Repository - CP/M Archive (mirror)
German mirror of the largest public CP/M archive.
ftp://ftp.mayn.de/pub/cpm/

Osborne CP/M Library
Archive containing software and documentation for the Osborne-1 and Osborne Executive.
http://www.guntersville.net/osborne/

The Unofficial CP/M Website
CP/M binaries and documentation, including the original source code and manuals.
http://www.cpm.z80.de

Wikipedia: CP/M
Encyclopedia article, with links to many related topics.
http://en.wikipedia.org/wiki/CP/M

ZCN
Operating system for Amstrad NC100 Notepad and NC200 Notebook; mostly compatible with CP/M 2.2; descriptions, documents, downloads. Free.
http://www.ibiblio.org/zcn/

Z-Node 51
Downloads and information for CP/M, Z-systems and Z80 computers. Includes a collection of articles and manuals, too.
http://www.znode51.de/indexe.htm