Strategy

the Forge - Ironman Guide - KingOfPain

Warrior "Styles" and Thoughts for Low-AC Warriors - Charis

An Analysis of Osmosis and Pain - Charis

A Study of Campfire - A guide to Firewall - KingOfPain

Caves: Monster AI and Layout Analysis - Cyrene

A Study of CB - Moriah, Nystul and KingOfPain

TeleFlash - Dr Disaster
TeleFlash Illustrated - Moriah

Chain Lightning Analysis - Moriah

Stair Death recovery and Spells during load - Nystul and KingOfPain

TeleKill Simplified - KingOfPain 

Hell 101 - a Guide for all Classes - Charis

An Analysis of Harmony and Fast Block - Charis

the D filez - Casting spells into another room
D filez II - Invisible Walls in the Butcher’s room and Hell
D filez III - Holes in the Catacombs and Hell - KingOfPain

Note:
- Some very useful tips can also be found buried within tales of adventures such as Cyrene’s Stealth Hunt and Nystul’s Geezer stories. Also, make sure to check out the original thread if an article has a link to it - Many more insight can be found from the reponses and discussions.

Variants and Ironman

Classic Diablo variants master list

Classic Diablo Ironman - Rules, Strategy, and Victory Reports

Battle Reports and Tales
Battle Reports and Tales

Links

The Asylum Message Board
Although Woody's Diablo Asylum is currently MIA, the Asylum Message Board lives on...

Dreams of the Sleeping God
Variants, Strats and home of the Forge, an Ironman guide by KingOfPain

Freshman Diablo
Lochnar[ITB]'s very useful site with lots of information on basic CD gameplay. It also has some information on Naked Mages, and Armin's "BNM Tips and Tricks" and "The Bare Necessities".

Sylvan Cliffs
Wheecht! LemmingofGlory's own site devoted to things pertaining to Diablo and Diablo 2. Many wonderful stories and variant rules too.