SOMA is described as 'unsettling story about identity, consciousness, and what it means to be human. Seek out the last remaining inhabitants amidst a world of horrors beneath the Atlantic ocean'. There are more than 10 games similar to SOMA for a variety of platforms, including Windows, Mac, Steam, Linux and Wine. The best alternative is Dead Space. It's not free, so if you're looking for a free alternative, you could try Cry of Fear. Other SOMA like games are Amnesia: The Dark Descent (Paid), Alien: Isolation (Paid), Layers of Fear (Paid) and Slender: The Eight Pages (Paid).
Amnesia: The Dark Descent is a first person survival horror game for Windows, Mac OS X and Linux. The game features a protagonist named Daniel exploring a dark and foreboding castle, while avoiding monsters and other obstructions as well as solving puzzles.
Layers of Fear is an foray into madness, a first-person horror game that sets a tone of fear and dread through provocative story telling as you explore a Victorian setting complete with mood-setting art pieces.
Slender: The Eight Pages is a free indie-developed psychological horror/survival horror video game for Microsoft Windows and OS X. The game is based on the Something Awful forums' creation, Slender Man.
Penumbra: Overture is the first in a series of episodic survival horror games developed by Swedish developer Frictional Games. It was originally intended as the first episode of a trilogy. Represent entire series of Penumbra games.
suspicious suicide. A chilling curse. A malevolent power. Finally, a wicked dark secret. This is Derceto, a legendary Louisiana mansion where, against your better judgment, you're drawn into a world of shadows to explore the darker side of Jeremy Hartwood's imagination.