IBM Links
zSeries and S/390 Technical Support. From here you can access technical data, fixes, and publications.
IBM's JCL manual online
IBM z/OS Internet Library
The IBM Redbooks home page. Redbooks are additional books written by various experienced IBM system programmers (not necessarily from IBM), and they tend to be more 'how to' oriented, including specific examples of things like VTAM and IOCP definitions.

General Mainframe Links
A general overview of IBM mainframes
Job Control Language (JCL) Introduction
A page full of links to mainframe-related information.
Loriaux's System/390 home page. A "Mainframers meeting point."

WAN Protocol Links
All You Wanted to Know About T1 Connections, But Were Afraid to Ask
Frame Relay overview
National ISDN Council
Overview of routing SNA by Cisco

TCP/IP Protocol Links
An extremely valuable overview of TCP/IP and the Internet
A list of well known TCP and UDP port numbers
The RFC (Request for Comments) regarding the Line Printer Daemon Protocol (LPD)
The RFC (Request for Comments) regarding the File Transfer Protocol (FTP)
The RFC (Request for Comments) regarding Data Link Switching (DLSw)

SNA Protocol Links
Introduction to SNA
Cisco technology Brief on SNA

DOS TCP/IP Software Links
FTP Software (now NetManage) Help Desk and Knowledge Base
FTP Software's (now NetManage) PC/TCP Kernel Software "Configuration Parameter Reference" document

Printer Links
Documentation on all popular code pages, including various ASCII and international code pages.
Lists the HP LaserJet PCL printer commands for basic page formatting and font selection..
Information, FAQs, and PDFs dealing with Xerox enterprise production printers, including all LPS, NPS, IPS, and EPS models. Also contains information on XPAF.
Xerox production printer user forums
Unicode Home Page. Contains information and documentation on Unicode.
The Unibook Character Browser is a freeware code page encyclopedia that allows you to look up all of the Unicode characters and their respective graphic symbols. It is handy for troubleshooting code page issues.

Windows NT/ 2000 Links
Microsoft Technet;en-us;q216206
Blue Screen or STOP Error Message Troubleshooting
Demystifying the "Blue Screen of Death"

SNA Server / Host Integration Server Links
Links to all Microsoft knowledge base articles on SNA Server
Links to all Microsoft knowledge base articles on Host Integration Server