I know, I know... We are almost in March, 2010. Still, I don't know if someone had come up with this topic already:
Why we don't make our own top 10 PC games lists, while having fun and helping GOG with our finest picks? ;)
(It's not PC-exclusives. Having a PC port, you are OK to add it here: you can add a game like Resident Evil 4 which has a PC port and console versions.
What you can't list is console-exclusives games, like Brutal Legend for PS3 and X360 only.)
In no particular order, I will start with:
Deus Ex
Operation Flashpoint
Civilization IV
Far Cry *
Half-Life 2
S.T.A.L.K.E.R.: Shadow of Chernobyl
Mass Effect
Diablo 2
Battlefield 1942
Fallout 3
*CHECK: already on gog.com.

Half Life 2
Portal
Prince of Persia Sands of Time* + Warrior Within
Silent Hill 2+3
American Mcgee's Alice
Beyond Good and Evil*
Penumbra Trilogy
Call of Cthulhu
Devil May Cry 3
Psychonauts*
Unwanted 11:
Metal Gear Solid 1+2
What can I say, Action Gamer is an Action Gamer.
I really wish those games will be added on GoG soon.

Anachronox
American McGee's Alice
Condemned
Diablo 2
Dragon Age Origins
Knights Of The Old Republic
Mass Effect 2
Portal
The Witcher
Warcraft III

Space Rangers 2
Deus Ex
Baldur's Gate 2: Shadows of Amn
The Witcher
Team Fortress 2
Psychonauts*
Max Payne 2: The Fall of Max Payne
Freedom Force
World of Goo
Sid Meier's Civilization IV

Portal
Company of Heroes
Dead Space
Max Payne
Osmos
Torchlight
Perfect Cherry Blossom
Toca Race Driver 3
Titan Quest
Audiosurf
Team Fortress 2
thats off the top of my head, it would probably be different if i took my time :P
EDIT: Hahaha im a moron, forgot its PC games :P

I thought it was supposed to be about PC games. Anyways, in no specific order:
Dragon Age: Origins
Deus Ex
Baldur's Gate II + Throne of Bhaal
The Witcher
Anarchy Online
Crimson Skies
Dark Messiah of Might and Magic
Sword of the Stars + addons
Arx Fatalis*
Prince of Persia: Warrior Within

PC-Games/ports, nothing else allowed

Yeh i realised after, changing it ;)

1. Deus Ex
the rest:
Fallout 3
Civilization IV
Oblivion
Morrowind
No On Lives Forever
World of Warcraft
Railroad Tycoon 3
Knight of the Old Republic
Medieval: Total War
too bad Alpha Centauri was 1999.

Hmm, I need to think here...
For now I'll just put three games on the list. More later.
Final Fantasy Tactics: War of the Lions (Note the version!)
Final Fantasy XII
World of Warcraft (especially the time between 2.4 and 3.0)

SPACE RANGERS FTW!

1)Witcher (Best all around RPG and massive fun)
2)Deus Ex (What is there to say?)
3)Team Fortress 2 (Team Based Shooter that kills anything else in its sub-genre)
4)Diablo II (Mindless fun that took years of my life)
5)Guild Wars (This gets this spot because of an actual fun PVP MMO that took sooo much time)
6)STALKER: Shadow of Chernobyl (I'm a real sucker for an atmospheric FPS/RPG)
7)Bloodlines (Best vampire game I have ever played and really knows how to use a good questing system)
8)Gothic II (The combat system makes learning the game actually very sweet and has a really nice sense of progression)
9)Mount & Blade (Combat system blew me away and still consider this the pinnacle of melee based combat)
10)Medieval II: Total War (Great for that strategy itch I rarely get)

Gothic
Deus ex
Mafia
Hidden and Dangerous 2
Balder's gate 2
Metal gear solid 2
Persona 4
The longest Journey
Psychonauts
no one lives forever

In no particular order:
Deus Ex
Mass Effect
Fallout 3
Knights Of The Old Republic
Medieval II: Total War
Morrowind
Company of Heroes
Vampire The Masquerade: Bloodlines
S.T.A.L.K.E.R. Shadow of Chernobyl
The Witcher

In no order :
Silent Storm
Rise of Nations
Europa Universalis 3 : Heir to the Throne
The Witcher
Team Fortress 2
Mount & Blade
Civilization IV : Beyond the Sword
World in Conflict
Company of Heroes
Combat Mission : Afrika Korps