Wikipedia: CP/M - Encyclopedia article, with links to many related topics. - http://en.wikipedia.org/wiki/CP/M
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
S100-Manuals - An archive of antique software and manuals related to S100 and CP/M hardware and software. - http://www.s100-manuals.com/
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/
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
Commercial CP/M Software Archive - A collection of CP/M software, including source codes and documentations. - http://www.retroarchive.org/cpm/
Z-Node 51 - Downloads and information for CP/M, Z-systems and Z80 computers. Contains a collection of articles and manuals, too. - http://www.znode51.de/indexe.htm
CP/M Main Page - Resource on technical internals, software, utilities, many links; by John Elliott. - http://www.seasip.demon.co.uk/Cpm/
CP/M Center - Information about CP/M and links to various CP/M resources. - http://www.gaby.de/ecpm.htm
The Unofficial CP/M Website - CP/M binaries and documentation, including the original source code and manuals. - http://www.cpm.z80.de
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
Osborne CP/M Library - Archive containing software and documentation for the Osborne-1 and Osborne Executive. - http://www.guntersville.net/osborne/