First, I advise the curious reader to listen to this talk by Al Seckel, and then take a look at the wonderful set maintained by Michael Bach. Read the fineprints below the illusions! That's the most interesting part - the references to the research papers.
Now, when you've done with all that - I wanted to show something quite powerful that I've found on the interwebs- something that can transform the brain's perception of the very world around you.
What to do: follow the link, maximize the flash window to take up the entire screen, and stare at the center for a couple of minutes. And then, before beginning to actively blink, look around and watch what happens to the world. So, open the flash here.