A favourite of mine is Bridget Riley

I particularly like this:
http://www.posterspoint.com/english/lamina-tabla.php?lamina=8957&ord=a&p...

And these:
http://www.artfacts.net/index.php/pageType/exhibitionInfo/exhibition/175...

http://www.ridinghouse.co.uk/publications/riley_reconn.php

Here's the Google image search:

http://images.google.co.uk/images?q=bridget+riley&ndsp=18&svnum=10&um=1&...

I'm not so keen on the black and white optical illusion/op art style ones though.