I love good silhouettes. They are almost always ambiguous. When dealing with a silhouette the subject can be looking toward you or away from you and the lines can be drawn in to show either perspective.
Which way is this lovely girl actually facing?

P2 - Beach 3
Flickr Artist: ZaksterNT
This is an ad for ramen in the causeway Bay Station in Hong Kong. I love this kind of art. I would love to see this from a angle that reveals how distorted this image needs to be to get this one perfect view.

Trompe d'oeil Ad
Flickr Artist: djwerdna
Here is a possible cover art for Radiohead's album In Rainbow. There is some nice anomalous motion going on in this art.
Nicely done!

inrainbows 800x800
Flickr Artist: pixelbaustelle
Here are a few excellently rendered impossible objects by Josh Sommers. Excellently done, thanks for allowing these via the "blog this" feature of Flickr. Josh talks about the first optical illusion presented.
"A variation on the 2 or 3 prong image, inspired by the work of Walter Wick, who managed to photograph his version using a partially reflective piece of glass and very careful lighting to create the illusion. "

Columns
Flickr Artist: Josh Sommers

Impossible Something
Flickr Artist: Josh Sommers

Impossible Posts
Flickr Artist: Josh Sommers

Two or Three?
Flickr Artist: Josh Sommers
Here is an interesting photograph. This photo was not manipulated with Photoshop or any other photo manipulation software.
How is this illusory effect accomplished?

Optical illusions
Flickr Artist: zm