I majored in psychology in college, so I found it pretty interesting to learn about how your mind plays tricks on you- check these out:
This optical illusion was one of the first I had ever seen.  Is it a bunny or a duck?
This optical illusion I only recently found on the web, and I honestly couldn't see it without a couple hints.  Do you see a bearded man with ivy on his head, or do you see a couple kissing?  Give yourself a lot of credit if you can differentiate the two images without help.  I'll put the clues at the bottom of this page. ~
This next item, though not entirely an optical illusion, can be difficult if you don't pay close attention to the instructions. 
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~Steps to see the man/couple kissing picture:
1. Treat the hair as nothing at first
2. Focus on the nose of the old man
3. The nose is the girl's face.
4. The eyes of the old man are the hair for the boy and girl. :)
More Sites on Optical Illusions:

Ryanbut

Optillusions.com

Eyetricks.com

AOL Illusions

UMDS
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