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.

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.

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

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?

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).

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?

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?

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.

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

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

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.