I play tested a new upcoming mod for HL1 not one hour ago and I was blown away for what must be the billionth time by this old little engine.
Was its name by any chance The Core?
Some of these were already mentioned above.
Minecraft: The complete freedom to explore, survive, dig tunnels or build things that weren't preset in the game world other than the one basic building block and so few arbitrary limitations, it's about everything I ever wanted from a game since I knew games existed.
Commandos 2: Men of Courage: My favourite from the Commandos series. The most finely tuned of them all, despite the sprites-based graphics, it has an excellent gameplay that allows for wildly different strategies, and despite favouring the stealth element you could still run in screaming and flailing a machine gun. I don't remember how long it is, but I'm sure it wouldn't matter how long it was, it would always feel short. It even has a multiplayer mode, which I always wanted to try but never had someone to play with.
Hitman: Blood Money: My computer can't run Absolution so I haven't played it, but it can't be better. Blood Money has the best gameplay of all the Hitman games, all of the good qualities and about none of the bad. Despite some odd architectural decisions everything has been carefully designed to allow lots of different strategies. Excellent game from beginning to end, and my only complaint is that despite all the money you earn, you can never buy a third syringe.
The Sims: My first ventures into the world of Architecture. Pretty limited in some aspects but it didn't stop me from sinking countless hours into the houses. Having the family live in the houses later didn't matter that much though
Wolfenstein 3D: An unbelievably three-dimensional game for its time, and my point of entry to the world of modding. Played a lot and used the level editor even more. Found bugs and maxed out the sprite limits plenty of times.
Special mentions:
- Antichamber
- Amnesia: The Dark Descent
- Sim City 4
- Fallout 3
- Castle Master