Slave Zero
http://en.wikipedia.org/wiki/Slave_Zero

I'll reiterate that just about anything from LucasArts or Apogee is a winning pick, they all get my vote.
Also could go for Lemmings, Civilization, Oddworld, Space Quest, Lost Vikings, You Don't Know Jack (not really 'old' I guess, but hard to find and not always stable on newer PCs)
And for one out of the ether, Microcosm. http://en.wikipedia.org/wiki/Microcosm_(video_game)
No doubt someone has mentioned something I haven't thought of, lots of these folks know what's good! :)

Wing Commander. Start with the old stuff:
- Privateer and Expansion (I'm sick of play the free 'fan port')
- Armada
- 1, 2, 3 (Kilrathi Saga in general)
- Price of Freedom. The best WC game ever.
- and why not prophecy.
If I could only pick 2 though I would have to go with Armada and Price of freedom.
Next...
Thief 1: The dark project
Thief 2: the Metal age.
Star Control 3. - I would add star control 2, but the open source 'Ur Quan masters' is near perfect already...
Master of Orion II
With those games I could finally throw out all my old PC's I keep around just for gaming... Well probably not, but I wouldn't feel as mad about it when my wife does.

If these haven't already been mentioned...
Final Fantasy 7 (PC Edition)
Final Fantasy 8 (PC Edition)
Cyberia

One more game came through my mind..
Albion!!
This is brilliant, completely underrated RPG created by Blue Byte. One of my first RPGs and one of the best IMHO. Brilliant story and a lot of fresh ideas. I would buy it instantly.

as a big Rpg fan , what about
The eye of the beholder 1,2, & 3
Death trap dungeon
and Decent to undermountain ?
Not played these in years.

Point and Click adventures please!
Monkey Island series
Grim Fandango
Loom
Sam & Max Hit the Road
Broken Sword series
Full Throttle
Then some others.
System Shock 2
Baldur's Gate 2
Blood Omen: Legacy of Kain
X-Comm
The Elder Scrolls: Daggerfall
Populous
Deus Ex

Ive have had a quick browse of the past 55 pages and did not see these if already been suggested then qudos to the posters these are great games to play :D
Fragile Alegience
Published by Interplay
Developed by Gremlin Interactive Ltd.
Black Moon Chronicles (Chroniques de la Lune Noire)
Published by Cryo
Developed by Cryo
Imperium Galactica
Published by GT
Developed by Digital Reality
+1 for Planetscape torment, altho i have it on cd already but more people need that game in their lives!!!!

So, I ran a ruby script on the first 55 pages of this list. Here's the games (and some publishers) that were mentioned 15 or more times:
system shock (144)
planescape torment (117)
lucasarts (74)
wing commander (70)
mechwarrior (64)
baldurs gate (64)
dungeon keeper (63)
ultima (61)
blood (56)
sierra (54)
monkey island (53)
syndicate (53)
xcom series (52)
space (52)
crusader (51)
origin (50)
xwing (48)
thief (45)
tie fighter (43)
grim fandango (40)
arcanum (40)
deus ex (39)
might and magic (37)
icewind dale (34)
carmageddon (33)
final fantasy (33)
interstate (32)
outcast (31)
earth (29)
homeworld (29)
microprose (28)
worms (28)
battlezone (27)
master of orion (27)
duke nukem (27)
gabriel knight (26)
full throttle (26)
master of magic (26)
interplay (25)
kings quest (25)
alpha centauri (24)
sanitarium (24)
day of the tentacle (21)
blade runner (21)
little big adventure (21)
commando (20)
diablo (20)
discworld (19)
theme hospital (18)
populous (18)
dune (18)
total annihilation (17)
quest for glory (17)
no one lives forever (17)
civilization (17)
jagged alliance (17)
dark forces (17)
quake (17)
lands of lore (17)
jedi knight (16)
police quest (16)
magic carpet (16)
doom (16)
privateer (16)
omikron (15)
age of empires (15)
alone in the dark (15)
warcraft (15)
heavy gear (15)
anachronox (15)
myth (15)
myst (15)
lemmings (15)
Edit: Fixed a duplicate for Might and Magic. The inclusive total was 37. Also, note that this list is not perfect. A hand count would be a tiny bit more accurate. But this is close, and a lot less work.
Edit: The count for X-Com was wrong since I was filtering at 5 characters. Should have been 52. Also, Lemmings should have been at the bottom of the 15's. Dune, Doom, Myth and Myst were also lost by the 5 character filter. Searched for final fantasy without a number and got 33. (I may make more edits as I make tweaks to the script.)

- Shadow Company: Left For Dead (Sinister Games)
http://www.gamespot.com/pc/strategy/shadowcompanyleftfordead/index.html

Very nice :)

Steel Panthers 3
Developer SSI
Battle Isle Series
Imperium Galactica 2
Published by ?
Developed by ?
Alien Trilogy
Ascendency (4X space Game) Early Dos days, I think even before DOS 6.0 or near
Kohan Immortal Soverigns
Age of Wonders: Shadow Magic

This one falls in the category of Redneck Rampage and Blood, and i miss playing it:
Shadow Warrior
Published by GT Interactive
Developed by 3D Realms

And a few more I just remembered!
Theme Hospital
Death Rally
&& My previous list still stands! :)
Diskworld series
Broken Sword series
and Zork series
Hehe, the website has got me remembering back to when i was younger and the games I used to LOVE playing for hours and hours on end!

As has appeared several times already:
Wing Commander series
Mechwarrior series
Syndicate series
Magic Carpet series
Heavy Gear series
Lemmings series (especially Lemmings 2: The Tribes)
However, too, have not gone through all 56 pages of this thread, but I'd like to voice a few titles that may or may not be listed:
Strike Commander with Tactical Operations
Battletech: Crescent Hawk's Inception
Battletech: Crescent Hawks' Revenge
Uprising
Star Wars: X-Wing
Star Wars: TIE Fighter
Bioforge
Ascendancy
Jane's AH-64D Longbow
Longbow 2
Deathtrack
Autoduel