The games you find truly groundbreaking, original, awe-inspiring and memorable

Civilization -- the original DOS version way back when DOS didn't mean "denial of service".

Master of Magic

Shogun 1 Total War -- groundbreaking tactical combat.

Now, I'll admit I haven't actually played this game, and it hasn't even been released.....yet.

But it shows promise for the first 40K shooter ever.

WARHAMMER 40,000: SPACE MARINE

That is a game that is going to be ground-breaking.

Oh, yeah, and I think Battlefield 2142 is pretty ground-breaking, as it introduced a totally radical objective-based game mode, "Titan Mode", wherein the enemy team's epynomous Titan had to be destroyed, whilst protecting your own.

Call of Duty.

Lords of the Realm 2

Caesar 3

Age of Empires (the originals and 2)

Lord of Magic

Freespace 2

Titan Quest

I still play these games on occasion and love them to death. Lords of the Real 2 holds the crown though. So dated and lame by today's standards but I probably spent a couple thousand hours in that game and plan on spending more. Good times.

Impressions Games did good work.

Warlords

Stars!

Warcraft

Duke Nukem 3d (first game I modded!)

Prince of Persia (first game with DRM, couldn't play cause I had a copy)

Also some 3d racer I can't remember the name of, you could build your own maps in 2d

And WOW, which was groundbreaking in its success

Edit: FF7

Excellent entries, yes, Freespace, how could I forget

As an avid miniature painter (Space Hulk), I am also looking forward to 40k: Space Marine

Lord of magic - damn, I have to try this one. Could not run it under Win7.

Also, I forgot - ANACHRONOX!!!

... and Wizardry 7,8

... and Black Crypt!

Civilization 2 - Civilization may have been the original game, but it was Civilization 2 that solidified the concept and in my opinion created that genre. Truly memorable. Others may of course disagree; I'm judging this from the standpoint of someone that was very young when the original Civilization was released.

Alpha Centauri - The first game, to my knowledge, that really let you create your own units, with a "real" 3d environment where terrain elevation actually mattered. Lots of great concepts that haven't been seen since. Who hasn't wanted to build an army of continent-sinking terraforming ships ever since? If Civilization 2 created the genre, Alpha Centauri should've revolutionized it.

Fallout/2 - Real CRPG. I don't really need to say more. Real choices, real effects; Should've kept going in this direction, but it didn't. Hard to go back to in my opinion, due to general "clunkyness" which wasn't readily apparent in the past. The finer points of the game still stands, however.

Svea Rike - This means nothing to most of you, but it was the original "Europa Universalis"-type of game. The third installment of the series was dubbed "Europa Universalis: Crown of the North" outside of Sweden and the rest is, as they say, history. Eventually gave birth to the entire Europa Universalis, Heart of Iron and Victoria series. The grandfather of historical grand strategy on the PC (is there any other?).

X-Com / Jagged Alliance - the grandfather and grandmother of squad-based tactical games. Both actually went on to spawn a number of great games, but there's not really been anything like them since the death of their respective series.

This is by far not an exhaustive personal list. There are several others that have already been mentioned, such as Warcraft, Minecraft and whatnot. Just wanted to chime in. I also went for games that I considered ground-breaking, not necessarily the best of their series or genre, nor my favourite games. For example, I'd *want* to place Baldur's Gate up there, but I can't actually find an excuse to do so, besides "It was fucking awesome". It had a great story with amazing pacing, I suppose?

Turrican 1 & 2 - best arcade shooter ever with great music from Chris Hülsbeck

F-16 Combat Pilot - great simulator back in the days

Gunship - fun helicopter sim

System Shock - this game really needs a worthy remake

Deus Ex - great atmosphere

Duke Nukem 3D - fun FPS

Freespace - combined with a powerful but hard to learn editor

Hard Nova - nice scifi RPG

Eye of the Beholder - my first RPG afair

King's Quest VI - First game I ever played, awesome story, and definitely memorable.

Civilization II - My introduction to the Civilization series, it sucked up much of my time during my childhood. Before I actually bought the game, I used to tag along with my dad when he went to Future Shop, found a computer that had Civ 2 installed and played it till an employee kicked me off.

Unreal Tournament - My first game I ever became involved in clans to play competitively (though I played instagib CTF).

Try running it in XP mode with a compatablility mode for an even earlier version of Windows (say 98). It will still have issues, as all the bugs that made it stop playing correctly in earlier versions of Windows seemed to just make it crash there.

Homeworld - possibly the most successful 3D RTS ever made and in turn the benchmark for the genre. It is also one of the best backstories and campaigns I have ever seen in a game, and the gameplay is excellent and very supportive of the story.

StarCraft and Command & Conquer - the grandfathers of the RTS genre, what more needs to be said?

Deus Ex - Most immersive game ever

Marathon - In times of DOOM this game had 640x480, looking up/down and a mind boggling storyline, speech over Ethernet for MP and recording of MP-Games

Beyond Dark Castle - Most favorite platformer of all time

Prince of Persia (mac edition) - Brotherbound got awards for this, 640x480 with 256 colors and lovely details graphics and animations.

Warlords - Best strategy game of all time, easy yet challenging, replay able for years without ever loosing its appeal

Golden Axe - If you knew it, you loved it

Wodan the Trial - Easy to learn, hard to master. AI is not very challenging, but against humans it's splendid

X-Com (1 & 2) - Best squad based strategy of all times (as someone already said)

Gothic 2 - Best RPG ever, perfect mix of arcarde/action and RPG. Good story, interesting characters and detailed handmade world design.

Warrior Kings - Battles - Lots of bugs/imbalances that lead to the most interesting strategies ever seen. Wonderful game mechanics and nice graphics for a strategy game of that time. Very CPU dependent, no Occlusion culling (greatest failure of the engine)

Sins of a Solar Empire - It had to be mentioned!! Perfect game design, give you the feeling to be an overlord of the galaxy

IL - Sturmovik 2 - No Flight Simulator ever came close to this level of realism.

Battlefield 1942 - First FPS to ever allow large maps and vehicle use (so many different types). I've never had that much fun in an FPS before playing the wake island demo.

And many others already mentioned!

i found perehelion for free on the web - however, it plays on an 'amiga' computer.  DOes that mean it wont play on current windows PCS?

The amiga is a defunct line of computers, and is different then the pc or the mac. It certainly would not run on a pc on it's own, but you might be able to download an emulator or something like that.

I was inspired to look up perehelion myself which seemed interesting. Never played it back in the day. I saw it compared to Black Crypt which I did play. I seem to vaguely remember some very fond memories about that game, but I can't remember much beyond that.

At one stage it was superior to the PC, but they didn't do enough marketing and soon got jumped by Nintendo, and eventually PCs became better anyway.

Best regards,
Steven.

Shattered Horizon  -  truly 3-D FPS.  Very cool idea sadly ahead of its time.  It requires DX10 and since Win7 was still new at game launch many still XP-gamers at the time passed on this GEM of an FPS.  Do yourself a favour and pick it up with a couple of friends.  LAN parties with this game are amazing!  Plus it has DEDICATED SERVER FILES for the public.  Have to love any game that does that these days....

R.U.S.E.  -  Chess evolved.  In my opinion it's like playing Chess crossed with Poker.  First real innovation I've seen in RTS in years.  As far as I'm concerned the unit markers could just as well be spaceships as the WWII theme is nothing more than "window-dressing" and doesn't actually factor into the gameplay.  In other words, solid gameplay was designed and then the "pieces" were dressed up!

Dune 2

Duke Nukem 3D

Freespace 1 and 2

Fallout 1 and 2

Sudden Strike

Portal

Alpha Centauri

X - COM

Heroes of Might and Magic 1

CounterStrike

Half Life

Original War

Yes Yes Yes, A Thousand Times, Yes!

Alpha Centauri - a strategy game I played far longer than any to date

Neverwinter Nights - BG2 and Planescape Torment are far better games, but the mods had me playing again and again and again

Lords of the Realm 2 - Yep, I played the crap out of this

Zork - Seriously tickled my funny bone when I was young (and still does)

Nethack - This game has more ways to die and more ways to play than every modern game on the market

The Oregon Trail - probably the only game we were allowed to play on the computers at school in elementary

Marathon - Bungie's classic; one particular "typing" class turned into a frag-fest once a week with the instructor usually kicking all our asses despite his youthful age of late 60s

Dragon Warrior - my first JRPG; its sad that most of the modern ones tend to cling to the same teenage emo trope 

Pole Position - the first game I ever owned

Pong

The first ever video game, IIRC.

I congratulate you, Shunmaha. You have remembered that humble origin for the triple-A blockbuster hits we are currently accustomed to.

Let none forget Pong!

For me, it would have to be Ultima Underworld. That game was a 3D first person dungeon simulator-RPG, back when 2D first person mazes were considered to be first person shooters. It had physics, back most first person games didn't let you look up. It had multi-elevation surfaces, sloped floors and curved walls, back when everything else was a rectangle.

So many good things happened because of that game. Heck, that's why we've got Bioshock, Thief and System shock and that's why there's a Skyrim on the way, and Deus Ex 3 on the way.

After that(same year actually), there was Ultima 7. I don't know of any game on the market today that allows that level of interactivity with the world, in that much detail.

I agree with a lot of what's up here already.  I'd add a couple from my own experience to the list...

Out of the Park Baseball - Everything that Football Manager is for soccer, OOTP is for baseball, minus the enormous European customer base.

Portal - I'm terrified that the second one is going to suck, thanks to how awesome and truly inventive the first one was.  Call it the "Dragon Age Effect."

Europa Universalis et al - Pick a variation: EU, Hearts of Iron, Crusader Kings, Victoria, etc...  If you're even remotely a history or strategy buff, chances are something built on the EU engine has sucked away hours and hours and hours of your life at some point.

(Edit: I see that Portal and EU are mentioned already.  Shows me for not reading carefully.) 

Also, I need to submit what might have been the best gaming experience I've ever had:

Shadow of the Colossus - One of the most artistic games ever made, what with the fantastic landscapes and the stunning colossi.  Brilliant, also, in that those are the only two things you really ever interact with.  No minions to wantonly slay, no history to explain the astounding scenery, barely a story (but what there is is more than enough).  And yet, the game was so complete, so cool, so moving, so challenging, and so epic that I can't think of anything that could or should have been done differently.

Was it groundbreaking?  Probably not, since it hasn't - to my knowledge - been replicated.  But original? Awe-inspiring? Memorable?  Absolutely.