First, as a warm-up puzzle, count the number of f's at:
http://www.smart-jokes.org/count-number-f-sentence.html
I'm going to show you a video clip, in which there are two groups of people, a white team and a black team, and you are to count the number of times that the white team passes or bounces the ball to each other. Do you have that straight? There are two teams of people, and you are to count the number of times that the white team passes or bounces the ball to other white team members.
If you've done this before, please refrain from talking after the video and let the others have the experience.
This is a visual perception test that is used in industrial safety training. Getting something like this wrong in the real world could lead to something bad happening, like having to fill out lots of paper work. So sit up straight, focus, and count the number of times that the white team passes or bounces the ball to each other. The video can be found at:
http://viscog.beckman.illinois.edu/flashmovie/15.php
Don't look at the following link until after you've looked at the video above, or else you'll spoil the fun. But after you've looked at the video you can find an explanation of it at: perception test explanation.
More visual cognition tests at http://psyclab1.psych.ubc.ca/~viscoglab/demonstrations/
If you haven't already seen it: the Online Card Trick.
A good collection of 75 Optical Illusions & Visual Phenomena
Even more Optical Illusions.
An amazing curveball illusion, winner of the 2009 Best Visual Illusion Contest (probably lots of cool stuff here).
Another list of interesting brain teasers.