Agree with you.This games must be on GOG.com. :)

Blood Omen: Legacy of Kain
Legacy of Kain: Soul Reaver
Two fantastic and atmospheric games, it's a shame that they haven't been released yet on Windows XP. I know, there are some fan-patches and Virtual PC, but I'm waiting for professional 100% working Windows XP versions.

Arcanum + Black Isle games are must-be in GOG.
Also I would like to see:
Metal Gear Solid: Integral
Final Fantasy VII & VIII
Sanitarium

Oldschool RPG such as:
Ishar series
Might an Magic series
Lands of lore series
Dungeon Keeper series
Dungeon master
Elder Scrolls series
Menzoberranzan

i second TIE FIGHTER, X-WING and Star Trek 25th Anniversary. What about it's awesome follow up, Judgment Rites?
Might i also add BLOOD and BLOOD 2? jabbing a zombie with a pitch fork, anyone?

Oh, I forgot a couple of classic games that, in my opinion, are a must:
Psygnosis:
Lemmings
Broderbund Software:
Prince of Persia
So many good times with those 2...
Edit to add a couple of sims that I really enjoyed back in the day:
Digital Integration:
Apache Longbow
MicroProse:
F-15 Strike Eagle (II and III)

Hammer of the Gods. It started the whole Heroes of Might and magic turn based grid battle system. Definetly a must have.

and i like to add
severence (blade of darkness)
iwar-2 (independence war, space sim, very very cool)
freelancer
eurofighter typhoon
battlezone 1 and 2

earthsiege 2 - great mech simulation game...it was from Sierra?

What nobody for Mr. Constipated look of the year aka Max Payne?

That was a great game, too bad it was totally underrated by the media.

How many titles can I type? :))))
UFO: Enemy Unknown
X-Com: Terror From The Deep
X-Com Apocalypse
Syndicate
Syndicate Wars
X-Wing
Tie-Fighter
System Shock
System Shock 2
Reunion (Best space strategy game ever made !!! )
Imperium Galactica 2 (also best space strategy game ever made !!! ;)))
Wing Commander series
Privateer
Privateer 2: The Darkening
Quarantine
Quarantine 2: Road Warrior
Harpoon
Alone In The Dark 1&2
Lands Of Lore 2
Millenia: Altered Destinies (by GAMETEK) - a game from which SPORE gets it's "new" ideas btw:)
Oh, and I saved the best for last:
"OUTCAST" (by Infogrames)!!!
Propably the best adventure game I ever played (and I must say that I've played a few:)
It gave me countless DAYS of thrilling playtime ever!!!

GT Interactive:
Blood + Plasma Pack + Cryptic Passage
SCI Games:
Carmageddon + Splat Pack
3D0/ Ubisot:
Might & Magic VI The Mandate of Heaven (one of the best RPGs I've ever played)
Adventure Soft:
Simon The Sorcerer I and II

Oooo Project IGI - I had totally forgotten about that game. I played that way too much back in the day. Loved it to bits. Great game.