This is a heavily cleaned & modified version of artur83's Penrose Triangle, which was a great design however not printable.
It is now enlarged (longest dimension 99.97mm) and printable on almost any FDM printer without requiring support. Walls have been thickened at the bottom to ensure a secure footing and to minimise excess overhang angles.
Best viewed with one eye closed - looking at it with 2 eyes almost ruins the illusion :)
NB: A new improved version has been released! http://www.thingiverse.com/thing:6513
Simply print, support not required!
(the one in the photo was printed on a Rapman 3.1 with Orbi-PLA, but it is designed to be printable on a Makerbot or Mendel or any other printer really)