The above reaction gif has been used so many times on Reddit, it's in the Hall of Fame:

http://karmadecay.com/results/u1978270

For example, it's been used to describe reactions to Game of Thrones:

http://www.reddit.com/r/funny/comments/1fks8u/game_of_thrones_book_readers_vs_nonbook_readers/

What is its origin? The television show Survivor.

It's Matty Whitmore from episode 9 of season 17 of Survivor: Gabon — Earth's Last Eden.

This is Matty's giant shit-eating grin when he realizes that the leader of the Kota alliance (Marcus) has now been voted out of the game.

Corinne Kaplan and Sugar Kiper express their disbelief.

More on Survivor Gabon: 

http://survivor.wikia.com/wiki/Survivor:_Gabon

You can watch the clip 20 seconds into this video:

I am seeing this gif more in gif mashups lately. Example:

Internet 4chan gif is Google Reverse Image Search's best guess as to the name for the gif.

Which is funny because searching for that phrase finds this:

Source:

http://www.reddit.com/r/funny/comments/oylfg/that_sums_it_up_pretty_well/

For those unfamiliar with 4chan, let's just say it's the kind of place to find everything from this...

...to this...

...which tends to be taken too far...

Eddie Murphy captures the spirit of the shock and smile:

And he's responsible for one of the greatest songs of the 80s, "Party All the Time":

On a completely different note, I found this 4chan costume in an Imgur thread:

Good times. Here's the thread it came from: http://imgur.com/gallery/usxUt/comment/110500007

Cue the reaction gif.