I've been reading some blogs on human consciousness and found a link to some fun perception teasers I enjoyed:
optical illusions I to IV
In the ones that trick perception into seeing different things (usually 2) in one same image (old and young ladies, duck and rabbit) I love going from one to the other really fast.
There are a bunch that explore Escher type of images involving either recursion or impossible architecture, I love those two. I guess the familiarity of recursion makes it fun as life is recursive itself (tree and seed, kid and parents, web and pages).