This is the main site on the net about ambiguous art.
Here you'll see a lot of clever artist's work.

The works of Escher, Orovitz, Shepard, Fukuda, Dalí and more...
 

"An ambiguous image can extend itself and continue the paranoic advance, til the presence of another idea be as enough hard to make appear a third idea, and so until the infinite. The number of recognized images is limited only by the intellectual paranoic skill" - Dalí -