Anyone know any good non-linear exploration-based FPS games in the same vein of System Shock 2?

I love System Shock 2 with a passion. FPS games with exploration elements and/or RPG elements are phenomenal. The kinda game I'm looking for needs to not be so linear (Halo and Half-Life are linear in this way), have a nice focus on being able to backtrack at any time, and explore. RPG elements are appreciated, but are indeed optional.



I've already played Metroid Prime, Borderlands, the newer Fallout games, Deus Ex, and Bioshock 1/2. I'm looking for something new now, my bros. I guess those games can be considered examples of what else could be similar, too.



Your answers are much appreciated!Anyone know any good non-linear exploration-based FPS games in the same vein of System Shock 2?
Try S.T.A.L.K.E.R. SoC if you havent already.