Some games I'd recommend on the RPG side of things (I'm leaving out the Infinity engine AD&D games, as I'm sure many others will recommend those):
Gothic 1&2 (if you don't mind the graphics), or Risen for a more modern game in the same style. (first/third person open world games)
VTM: Bloodlines (third-person, semi-linear, but plenty of choices on how to play your character and accomplish the various quests, great atmosphere)
Deus Ex (FPS/RPG hybrid, semi-linear like Bloodlines, plenty of choices on how to handle quests, great atmosphere)
Divine Divinity (isometric, open world)
Divinity 2 (Divine Divinity sequel, third-person, semi-linear but plenty of freedom to explore within the large areas you move through)
Drakensang and it's recent prequel The River of Time (third-person, party-based, semi-linear but with lots of exploration, plenty of clever ways to handle quests if one is so inclined)
On the adventure game side of things two games I'd highly recommend are The Longest Journey and Sanitarium.
Beyond Good & Evil (Are there any other games like it? It was definitely one of my favorite games)
Psychonauts is fairly similar in the overall style and feel of the game. It's also a game I'd
highly recommend (and I was first made aware of it after asking the exact question you just asked).