Flipside tells the tale of an escaped insane asylum patient who has a split personality. He sees everything in cardboard, and cheerful or depressing depending on which personality he's using.
This is a Half-Life 2 mod, and contains only one short level. The game was previously featured in PC Zone, reviewed by Mod DB, and was submitted as a student entry in this year's IGF competition. Has decent music as well, sort of a Caribbean feel. (mmm, fondues...) And yes, there are plenty of working mirrors this time.