I remember a while back there was some website or TV show whose subject was "1000 Places to Visit Before You Die" or something like that. So, as gamers can YOU think of the games someone should definitely play before shuffling off the old mortal coil??
I will start:
Fallouts 1 and 2 (The current Fallout 3 is quite optional)
Morrowind
Planescape:Torment
Wizardry VII Gold
Ultima Underworld 1 and 2
Thief (pick one they are all good in their own way)
Splinter Cell Chaos Theory
Baldur's Gate Series
All of the Goldbox SSI games.
Ultima 6-7(with all addons)
Deus Ex (sequel to remain optional)
System Shock 1 and 2 (of course)
Wolfenstein 3d (because some games wouldn't be here without it)
I'm sure I can add more but I want to see your opinions. Fire away!
Edited for grammar correction.

Pong, at least once to see where it all started.
There are earlier examples of video/computer/electronic games, but Pong gets all the recognition.

Yes! You're right. How could I have forgotten about the first known video game.
A bit off topic: I wonder if the first machine to play Pong is kept in a museum somewhere. If it isn't it should be just for the historic value.

Deus Ex
Quake III Arena
Unreal Tournament GOTY 99
Half-Life 1
Sven Coop, with a few good friends. :)
Breakout or any clone of it.

The Computer History Museum
Has various Pong related material, can't find the original machine there though.

Tetris, I mean c'moon!
Street Fighter 2
Super Mario Bros (NES) and 64
Command and Conquer
Dune 2
Civilizations
Starcraft
Warcrafts including World of
Zelda's (link to the past and ocarina of time atleast)
Final fantasies
Chrono Trigger

Ah, too many games, too little time :-(
I would like to be able to play them all, finish only those which are best in my eyes, but I really want to play every game that was ever created.
Are there games in the afterlife? If not, I'm staying here.

Doom, Duke 3D, Half Life, Quake 3, Unreal
Myst, Total Annihilation, at least one GTA and one Final Fantasy (VI / VII), Fallout, Knytt/Knytt Stories
I have lot more of games in mind but I don't think I need to put them in this list.
Some games I haven't play yet but need to, and should before I die (as I was recommended to) : Ico, Planescape Torment (still waiting to have the time). Dunno about others, I don't remember :>

Freespace 1-2
Fallout 1-2
Monkey Island 1-2-3
Grim Fandango
Planescape Torment
Portal
Uplink

Maniac Mansion
Zak McKracken And The Alien Mindbenders
Day Of The Tentacle
Loom
And a few obvious missing classics:
Pacman
Space Invaders
And a few not-quite-so-old classics:
Defender Of The Crown
Syndicate
Alien Breed SE
And a few of my personal (old) favourites:
Wizball (C64)
Nebulus (C64 or Amiga. US title: Tower Toppler)

I didn't....it's in my list dude XD.

mea culpa :) but diablo isn't ;)

Abe's Oddesey / Exodus ( By far best platform game ever )
Command and Conquer : Renegade ( Only the multiplayer )
Company of Heroes
Dark Messiah ( Great action RPG )
Fable ( The first one )
Final Fantasy Tactics
Guildwars ( Best competitive RPG )
Hitman: Blood Money
Jet 'N' Guns
Mafia ( Great Story! )
Mechwarrior / Mechcommander ( Any! )
Metal Gear 1 & 2 ( Best in the series )
Return To Castle Wolfenstein Enemy Territory
Savage II ( Now Free! )
Super Mario Bros. 3
Trackmania Nations Forever ( Free too! )
Tyrian 2000
Unreal Tournament 2004
Warcraft 3
All I could think of at the moment, piece by piece the best games around.

Just saw this : Euro Truck Simulator. Yeeehaaawww!! Adding it to my list....