Timecrimes (2007) Imagine accidentally traveling back in time, several times, in order to flee from yourself, because you are pursuing yourself.
Puzzlehead (2005) An exploration in roommate psychology, only in this case the roommate is your android clone.
Primer (2004) Four garage scientists who are relentlessly attempting to improve on a refrigeration system, among other things, accidentally create a time machine.
Code 46 (2003) In a not-so-distant future, biotechnology and genetics shape geopolitical boundaries, determine mating compatibilities, as well as control many other aspects of life.
Final (2001) Calmly mesmerizing post-pandemic psychological thriller. Denis Leary and Hope Davis are awesome in this.
The Quiet Earth (1985) A man awakens to find himself completely alone in the world.
Stalker (1979) A guide leads two men through an area known as the zone to find a room that grants wishes.
Solaris (1972) A psychologist is sent to a station orbiting a distant planet in order to discover what has caused the crew to go insane.
Forbidden Planet (1956) A starship crew goes to investigate the silence of a planet's colony only to find two survivors and a deadly secret that one of them has.
5ive (1951) The world is destroyed in a nuclear holocaust. Only five Americans survive, including a pregnant woman, a neo-Nazi, a black man and a bank clerk.
Destination Moon (1950) One of the first science fiction films to attempt a high level of accurate technical detail tells the story of the first trip to the moon.
Conspicuously, nothing from the 1960s or the 1990s made it to this list. If you know of any under-the-radar sci-fi gems from those decades please comment about them!