DiabloII.net
A very comprehensive site on Diablo II, the host of some excellent guides and information as well as the Diablo II Marketplace, an online auction site for in-game trading.

The Lurker Lounge
Formerly the best Diablo site on the internet, the Lurker Lounge has since closed it's doors, but remains as an archive. May it rest in peace.

Sirian's Diablo2 Page
A well known player of variant characters, Sirian's site contains well written reports on many of the characters he's played through the game. Also available is his excellent guide to the Sorceress' Fire skill tree.

The Arreat Summit
Blizzard's official Diablo II site. No better source for statistics, but I would look elsewhere for strategy and tips.

The Diablo Strategy Compendium
A recently opened site containing both forums and hosted strategy, the Compendium appears to be a site worth visiting for guides or discussion.


Multi-User Domains (MUDs):

chainsmokingdryad
Despite the unorthodox name, this site actually contains some excellent MUD resources, including Mish's very well written Mobprogram guides.

Mu's Unbelievably Long and Disjointed Ramblings About RPG Design
A massive compilation of game design theory by author Musashi. Excellent theoretical background on the ideal online world.

Raph's Website
The homepage of Raph Koster, one of the prominent figures of internet game design. All the writing posted on his site is worth reading over at least once.

The Mud Connector
This site provides over 1700 MUD listings, as well as a forum and several hundred resource links. The starting spot for anyone interested in MUDs.


Shadowrun:

Blackjack's Shadowrun Page
In my opinion the most insightful author of Shadowrun material on the web, Blackjack has presented many articles over the years on ways to increase the quality of one's roleplaying and gamemastering. Also featured as a columnist on The Shadowrun Archive (below)

The Shadowrun Archive
While the general quality of the submissions to the archive is lacking, the hosted columns are all worth a read for the excellent theories in gaming they present.


Miscellaneous:

ASCHII Art Index!
A site containing links to several libraries of text-art, including some where you're able to generate your own using the provided program. Can be highly useful for applications where true graphics may not be used; for instance, the sign-in screen of a MUD.

Darwin Awards
A highly amusing site, officially described as follows. "The Darwin Awards commemorate the remains of individuals who contribute to the improvement of our gene pool by removing themselves from it in really stupid ways."

FreeTranslation.com
An always useful website featuring translation between numerous languages.

Lee's (Useless) Super-Hero Generator
This site provides an excellent source of superhero and alias names for almost any game, and can even be used to generate the weapons, vehicles and source of powers for your superhero..

List of French Names
Featuring both male and female first names and a wide selection of last names, this site is highly useful for the creation of any RPG character with a French sounding name. Even provides a program to randomly pair a first and last name inorder to create a complete name for you.