My favorite one: (Thanks John)
Stephen King's "Hand" - Follow the instructions carefully!

1. Leave your hand on the mouse
2. Double click the symbol below (it's virus checked)
3. Concentrate on the dot in the middle of the screen and count to 30 (It's very important to watch it for 30 seconds! Otherwise it won't work!)
4. Now look at your hand on the mouse
5. Don't scream!

Click here to download the file necessary for this!

Old woman...or young girl?

Hint: the old woman's nose is the young girl's nose and chin.

Man playing horn... or woman's silhouette?

Woman in vanity... or skull?

Hint: move farther a bit from the screen and blink to see the skull or the woman (looking at the mirror)

Two faces...or one?

A Rabbit.... or a duck?

Hint: the duck is looking left, the rabbit is looking right

A face of a native american... or an eskimo?

Is the book looking towards you or away from you?

Do you see some squares or rectangles?

Hint: This is nothing more than a bunch of lines going in every which direction, but the way our mind interprets these lines are totally different!

Do you see an old man with ivy leaves around him, or do you see a couple kissing?

Hint: This is probably the hardest illusion that I have ever seen, so it may take some time to discover.

Do the sides of the square appear to be bent in?  

Believe it or not, they are as straight as they come.  If you do not believe me, take a straight edge and see for yourself!

Does there appear to be fuzz where the white lines intersect?

There is nothing there!  Now, I want you to get two sheets of paper.  Then I want you to cover the entire grid except for four black boxes, and one intersection (helps if you do this in a corner of the grid).  Now, where did the fuzz go?

Just stare at center of this illusion for several seconds, be relaxed, and try not to blink if possible...if you are not careful, your ENTIRE WORLD might start moving!  
Don't say I didn't warn you!!!

Do the two vertical lines in this illusion appear to be equal in length?

Well they are!  If my word is not good enough for you, go and get a ruler and check for yourself!  They are exactly the same size!

Where does this triangle end???

Man, this one could get ugly!

Do you see a vase?

Or do you see something more.  Hint: Two people!  Look Closely!!!!

How many colors do you see?

When you first look at this, some might say they see 3 or even 4 colors, but there are 2!  Red and green!  People usually think they see 2 shades of red, but there is only 1.  Look closely and you will notice on one side that white boxes surround the red boxes, and on the other side, green boxes do!

Ask yourself, Which circle looks bigger?

The one on the left, or the one on the right? No, Wait, LOOK AGAIN!! They are the SAME SIZE!!

Inside or out?

Click here for more - Page 2