The pictures are copyrighted, so I won’t copy any of them here, but there are several more illusions by “Akiyoshi Kitaoka”:http://www.psy.ritsumei.ac.jp/~akitaoka/shikisai2005.html at his website. For a really striking contrast effect, scroll down to the “Green and Blue Spirals” image.

UPDATE: And here is the page of illusions by “Beau Lotto”:http://www.lottolab.org/Illusions%20page.html at UCL. Thanks to Jamie Dreier for the link. Again the images are copyrighted, so I won’t cut and paste any, but they are utterly remarkable. (Note that one of them is a brown/orange ‘illusion’ that, as Daniel Nolan noted in the previous comment thread, we might want to think twice about saying is illusory.)