From here: http://www.telegraph.co.uk/news/newstopics/howaboutthat/3520448/Optical-Illusions---the-top-20.html

Number 7 is an old favourite. Cut and paste B next to A and you still can't believe it, surely.
http://www.telegraph.co.uk/telegraph/multimedia/archive/01121/same-color-illusio_1121075i.jpg

Number 11...well I traced the circles with my fingers but it still looks like a spiral to me.
http://www.telegraph.co.uk/telegraph/multimedia/archive/01121/597px-fraser-spira_1121083i.jpg