Movies like this?

The Butterfly Effect
Altered States

The recent Tom Cruise movie, Edge of Tomorrow, sort of fits this category.

VIRTUAL NIGHTMARE (TV Movie)

Cant believe every one forgot Brainstorm (1983). way ahead of its time.

Brainscan (1994)
Interstate 60: Episodes of the Road (2002)
Cloud Atlas (2012)
Sphere (1998)
The I Inside (2004)
The Nines (2007)
The Jacket (2005)
The Quiet Earth (1985)
The Scribbler (2014)
The Zero Theorem (2013)
Enter Nowhere (2011)
Stay (2005)
The Machinist (2004)
The Hole (2001)
Passengers (2008)
1408 (2007)
The Deaths of Ian Stone (2007)

Kinda:
Mindscape (2013)
Gothika (2003)
+1 (2013)
Premonition (2007)
The Dark Hours (2005)
The Voices (2014)
After the Dark (2013)
Event Horizon (1997)
Extracted (2012)
Flatliners (1990)
Infini (2015)
Haunter (2013)

Some great recommendations here. I also love movies that question the nature of reality. My favourites are:

Triangle (2009)
Inception (2010)
Total recall (1990)
Edge of Tomorrow (2014)
The Matrix (1999)
Fight Club (1999)
Stay (2005)
Existenz (1999)

Although not necessarily related, I would also recommend the following to people that like those films:

Session 9 (2001)
Dead Man's Shoes (2009)
Predestination (2014)
The Prestige (2006)
The Cabin in the Woods (2011)
Frailty (2001)
Frequencies (2013) aka OXV: The Manual
The Skeleton Key (2005)

Cypher
Dark City

both Solaris
but Tarkovsky´s Solaris is way better

at the end the living sea start recreating a part of the earth thus creating some sort of simulacron

Cure kuya 1997
also make you question who you are

Open Your eyes (not mentioned in this thread) is slighty more realistic than hollywood Vanilla sky