Me Gusta is a rage comic face, often used in response to an awkward or weird situation. Me Gusta is spanish for "It pleases me" or "I like it". Originally used in perverse context, but has broadened over time.
The first comic on Reddit was posted on March 18, 2010 by Redditor “me_gusta”. In a comment he claims he discovered the image on 4chan before posting it to Reddit.
Also has spin-offs such as:
Creepy Me GustaEdit
Me Gusta Mucho or Creepy Me Gusta is a variation of Me Gusta with the mouth changed to a smile. It is used to show extreme pleasure as opposed to simple pleasure, or as a creepy form of Me Gusta.