System Shock 2

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Yeah, sacrificed what little free time I had and played it all night long. Never got to play it before, because I just couldn't get it to work for more than 5 minutes on my PC. But the GOG version works like a charm!
So far, I'm loving it. It's creepy, it has open levels, just the right amount of combat and the story is quite engaging, too. I also find it interesting how, despite the completely different settings, I was instantly reminded of the Thief series while playing - of course without knowing yet that they were both created by the same developer.

You're probably then already aware of the fact that both games use the same engine (Wikipedia says SS2 uses an "Enhanced Dark Engine") and the same animations which is why it reminds of Thief.