My 'intranet' : Links

http://www.serial-networking.fsnet.co.uk/
The complete guide to Serial Networking. Concentrates more on networking without a network card (yes, it can be done!) but is a useful reference for setting up TCP/IP.
http://www.foggy.co.uk/networking/
Foggy's networking guide (good)
http://www.vigay.com/cgi-bin/search?a=networking
Paul Vigay's networking guide (brief but concise)
http://www.argonet.co.uk/users/pirate.king/network/network.html
The Pirate King gives us walkthroughs of setting up a PC (Windows ME), OmniClient, and LanMan98. Detailed, but graphically intensive (each step is explained by a full 640x480 screenshot).

VNC

VNC allows you to link two computers graphically, so you can use the Desktop on a PC (or vice versa). I've not yet had any success serving RISC OS to a DOS box. :-)
For more details, visit:

http://www.orl.co.uk/vnc/

Other useful things

http://freenet.barnet.ac.uk/freenet/c.poole/
DeltaNet, for server applications - this is where you'll find an ftp server for RISC OS.
http://www.teletec.f9.co.uk/networking/riscos/programs.htm
Replacements for OmniClient, require the LanMan/LanMan98 modules though.