I found this image on a rather large optical illusion site.
The interesting thing wasn't the below illusion, but the fact that they just didn't get it. They had this wonderful description of how this wasn't an illusion but was just distorted squares reminiscent of some of the spiral optical illusions that are out there.
The truth however is that this is a great optical illusion. The above "distorted squares" aren't distorted at all. Each side is perfectly straight meeting at a 90 degree angle with the other perfectly straight sides.
To prove this to yourself use a sheet of paper and place the edge against the screen. Another way of seeing the truth about optical illusions where the lines appear crooked or bent is to get up close and personal with your monitor. Go to the edge of your monitor and look across the screen, you will see that the lines are indeed straight.
I won't mention the site that I found this on out of respect for their otherwise great site. Also when I went back to the site I couldn't find it again so I may be confused about which large optical illusion site it was.