Nox (the better single player Diablo, with some incredible game mechanics, even looking at it today)

Hexplore

Imperium Galactica

Giants (this game ran like shit on every age appropriate PC, I'm kind of wondering if the engine can even run without stuttering, but it's a fantastic game)

Gothic 1 (alive open worlds are not that new and exciting anymore, but this game has a lot of charm & an amazing sense of exploration)

The Longest Journey

Knights and Merchants (combat strategy game, the later levels are combat only and it's very HARD)

Rage of Mages 2

Chrono Cross (probably the best jrpg of all time, but the combat system scared away many)

These are not really forgotten & qualify for being a cult classic, but merely they are old titles that the new kids have never touched:

Baldur's Gate 1-2

Morrowind (so much better than Skyrim, it's not even close)

Pharaoh & Caesar 3 (the city builders, there is a recently released HD remake for Pharaoh)

Oddworld: Abe's Odessey (Much better art style & direction than in the still great remake called New and Tasty)

Jazz Jackrabbit 1 (Sonic feels soulless compared to this)

Settlers 3

Chrono Trigger (It's not really a cult classic, because eventually all jrpg fans play this, right, RIGHT?)