You may well have seen this before, but squares A and B are the same shade of grey. Go to Edward H Adelson's website for an explanation. Photo editing software is good for showing how it's done.
The Lilac Chaser is another popular optical illusion.
This less popular illusion is a tricky one. Apparently if you stare hard enough at the image you may catch sight of a Prime Minister. I can't see it myself - all I see is a PR hack for vested interests. Maybe rose-tinted specs would help.