You probably don't even know Sanitarium. It was released in 1998, when adventure games were very much on the wane, at least in terms of commercial success. But this tale of an amnesiac mental patient living through a world forged by an unstable imagination - like Psychonauts meets Planescape: Torment - is as good as any of the more widely recognised genre greats.
And now, thanks to Good Old Games, you can download it (DRM-free!) for just $US10.
Sanitarium [Good Old Games]