http://cognitivefun.net/
It has a number of pretty interesting cognitive tests and "mini games", and you can even track your results if you wish. I found it when I was looking for a web-based "n-back" test. It's something I have started doing on my iPad, and it's oddly addictive. (That is, once you stop failing even at 2-back and begin noticing improvements!
)I actually wonder how much (if at all) of the improvements gained by these exercises transfers over to Go. I believe there may be a correlation between n-back and reading, but I'm still experimenting! (Unscientifically, naturally!)