These are available on Steam or Good old Games. In no particular order/typing out as they come to me! (Note: These are non-MMO games. I have several MMO games that I love too!)
Septerra Core: Legacy of the Creator
Diablo 1 and 2 (with expansion) One of these is usually on any computer I own.
Dungeon Keeper 2 Usually installed. (I love the skirmish and My Pet Dungeon modes!)
Portal 1 and 2
the Myst series (posted pics from those games here to show what I would love as a fantasy setting) I love the storyline of this and the archaeology discovery feel as you learn about D'ni culture!
The Elder Scrolls games (Morrowind, Oblivion, Skyrim)
The Enchanted Cave 2
Dungeon Siege 1 (and expansion) 2 and 3
Icewind Dale
Baldur's Gate 1 and 2 (have not played 3 yet)
Soulbringer is an epic story and has a unique combat system!
Stacking
Machinarium
Botanicula
Vampire The Masquerade: Redemption
Vampire The Masquerade: Bloodlines
Torchlight and Torchlight II
Clive Barker's Undying
The Longest Journey and Dreamfall (Though Dreamfall is kinda sad...)
Darkfall: The Journal Darkfall 2: Lights Out and Darkfall 3: Lost Souls
American McGee's Alice and Alice: The Madness Returns
Unreal Gold (and return to Na Pali) as well as Unreal Tournament 3: Black edition (My favorite weapon is the rocket launcher! BOOM BABY! I can hit you while in the air!)
Two Worlds II
Trine and Trine II are nifty platformers!
That's what I can pull up atm I know I've left some out!
Edit: How could I forget these! vv
Spore Creepy and Cute with Galactic Adventures (Last I knew my Feathered Bog Jumper was still in game! *brag* It's over ten years old now! I'm KatKMeanders in Spore.)
Sims 2 with Nightlife, Pets, and FreeTime
Terraria (but been a bit since I've played I'm behind quite a lot)
Minecraft
Defense Grid: The Awakening (tower defense goodness with a GLaDOS mod)
Stardew Valley