I thought I'd go slightly deeper than BugBozer
3DT - 3 Diamond Tower -- A socketed Tower Shield/Pavise with three perfect diamonds.
3PD - 3 Perfect Diamonds -- unlike 3DT, this could be any shield, not only a Tower/Pavise. Bone Shields for lower requirements, or Large Shields for faster run/walk than when using a Tower/Pavise, for instance. Can, of course, still refer to a Tower/Pavise.
AR - Attack Rating
AS - Arcane Sanctuary
ATD - Attacker Takes Damage
Baba - Barbarian
BO - Battle Orders
BG - Blood Golem
BG/IM - The bug that will let your Blood Golem steal life from 33% of the thorns damage inflicted to enemies by Iron Maiden. Basically, it'll let your Blood Golem be invincible against melee enemies. Some call it a bug, some call it a feature. Most call it cheesy.
BH - Blessed Hammer
BH/Conc - The bug that lets Might and Concentration boost Blessed Hammer damage. Like BG/IM, some call it a bug, some call it a feature.
Bremm - Bremm Sparkfist, unique council member in Durance of Hate level 3.
BW - Blackwall OR Bone Wall, depending on context.
Cage - Rattlecage
CD - Classic Diablo (Diablo 1)
CE - Corpse Explosion
CEB - Cold Enchanted Boss or Cold Enchanted Bastard, whichever you prefer
clvl - Character Level
CM - Cold Mastery
Coif - Coif of Glory
Conc - Concentration
CS - Chaos Sanctuary
D2 - Diablo 2. Duh. :)
D/A/E - Dodge, Avoid, Evade (Amazon skills)
Dmg - DamageDoE - Den of Evil
DR - Defense (rating)
ES - Eastern Sun (D2 mod) OR Energy Shield, depending on context.
Exp - Experience
FEB - Fire Enchanted Boss or Fire Enchanted Bastard, whichever you prefer
FG - Fire Golem
FM - Fire Mastery
FW - Fire Wall
Goblin - Goblin Toe
HC - Hardcore
HCMTF - Hit Causes Monster To Flee
Heph - Hephasto The Armorer
HF - Holy Freeze
ilvl - Item Level. Used when determining which affixes should be availible on a magic or rare item.
IM - Iron Maiden
IS - Iron Skin OR Increased Speed, depending on context.
ITD - Ignore Target's Defense. VERY GOOD and uncommon modifier, only found on sceptres, wands, staves and the unique Scimitar, Skewer of Krenitz.
Izzy - Izual
LEB - Ligtning Enchanted Boss or Lightning Enchanted Bastard, whichever you prefer
LM - Lightning Mastery
LR - Lower Resists
Meph - Mephisto
MS - Multishot (either the skill or the monster attribute)
MSLEB - Multishot Ligtning Enchanted Boss
MDR - Magic Damage Reduced by xx.
PD - Perfect Diamond OR Poison Dagger, depending on context.
PoL - Potion of Life (unique quest reward in act III)
PKK - Player Killer-Killer. Kills PK's.
PvM - Player vs. Monsters
PvP - Player vs. Player
SC - Softcore, as opposed to Hardcore.
Skin - Goldskin
slvl - Skill level. How many points you have in a certain skill.
TK - Telekinesis
Wall or WotE - Wall of the Eyeless
Desynch - When you are in two different places on client and serverside. You'll see yourself in one place, but in reality you're one screen away, being beaten to death by a Fallen. Caused by packet loss.
Double Draw Bug - Fires in D2 are drawn by stacking lots of sprites in a REALLY STUPID way. The sole reason FW, Blaze and Meteor lag so much, why Fire Skills suck, and why most Sorceress fans want to see Blizzard's programmers drawn and quartered.
Hidurare - A rare with 5x durability. These rares are created when the game tries to spawn a unique item, fails (because a) The item is exceptional, or b) The unique already exists in the game), and chooses to drop a rare instead. The thing is that the rare still gets 5x durability, like all uniques have.
Power Moron - As opposed to a Power Gamer, he is someone who plays all the most effective play style (tweaker sorc, WW barb, MS 'zon etc.) but has little or no understanding of the game mechanics. Generally considers himself to be the best Diablo II player on b.net, since he figured out how to read some guide on "How to build the most effective Barbarian". Power Morons make up 80% of battle.net's population.
Power Gamer - Someone trying to build the most effective character - by testing, experimenting, reading up on how the game works, looking for cheap exploitable skill combos etc. Quite rare nowadays, since after more than 6 months the most effective playstyles are already discovered (as far as we know - I could be wrong).
Reping - Going to town for a repair. Very common among Lance barbs, as lances have 25 durability (unless they're hidurares).
Soloing - Playing alone in a large game.
Variant - A character designed not to be super-effective, but to be challanging. It's done by imposing some restrictions on your character. For instance, a Sorceress using Lightning Skills ONLY.
WP trap - A "trap" set up at a waypoint. Example: You go through the River of Flame waypoint. As you "wake up" on the other side, you're already dead. Whoopdie shit, someone parked Hephasto right next to the WP. Not A Good Thing when you're in Hardcore. Can also be a player that goes hostile to you as you leave through the waypoint, and as you "wake up" on the other side there's a hostile PvP-built amazon standing next to your Sorceress. Not a good thing either.