Here are some very useful links for Blizzard's Diablo I & II. This is not a large list of links, but they're hand picked and, in my humbe opinion, represent the best and most important you can find about the game on the net.

[Note: The linked sites below will open in a new window each.]

Diablo I (Classic Diablo)

Blizzard's Diablo Strategy Forum [Linked now: Blizzard Forums List]
...is no more. It was closed in late 2002, thus having lasted for about seven years. It used to be the home of all Diablo strategy experts, code readers, testers, bug detectors and the like. Blizzard now recommends their "War Room" for Diablo I related questions. Good luck. (Note: check the Lurker Lounge Forums instead)

Jarulf's Guide to Diablo and Hellfire
The famous #1 of all Diablo guides. Has all the data, facts, formulas, and thus almost all the answers to any of your questions about the game. Download it as a WinWord document from Jarulf's new host, the Lurker Lounge.

Lochnar's Freshman Diablo
a very valuable source. Lots of small to mid-sized guides written by Diablo and Battle.net veterans. Newbie or expert... VISIT ! Some of the contents can be found on this site as well.

Claudio's Page
Info on DI (mostly on Rogues, some general stuff too) and DII (why it sucks so much =), good guides. Claudio is the creator of the CAT-Rogue style of playing: very sophisticated!

Diablo II

DiabloII.net
Large, semi-professional site run by Flux (plus some others now). Ambiguous, somewhat overwhelming... but a must see.

The Lurker Lounge
Bolty's Lurker Lounge is "the Diablo Strategy source" for DII and Classic D. Of course, he's now hosting his own DI Warrior Guide too.

The Arreat Summit
Blizzard's Geoff Frazier made this site for Diablo II: Lord of Destruction. Huge effort. Loads of Data and information.