Alex Tabarrok links to a pretty neat optical illusion, which gave me the spur for this follow-up. If you think these may be computer tricks, rather than you brain playing games with you, simply print out the page - the illusions will work just as well.
And the best one for last (you may want to click on the image to maximise its size):
2. Then look at a wall near you - a bright spot will appear
3. Twinkle a few times and you will see a figure
Who do you see? A miracle or what?
Sorry, no sources for these (forwarded to me in emails etc), other than the 'rotating snake illusion' (second from top) which is due to Akiyoshi Kitaoka. If you liked this post, his webpage has a very large number of illusions to occupy your mind with.
And if you feel like digging deeper, check out my very-worthwhile-looking latest purchase - Mind Hacks (the book's blog is here). No unconditional 'buy' recommendation yet as I haven't had time to read it, but stay tuned.
UPDATE: One more from an older MR post:
Don't believe it? See here.