I have been glued to my computer since discovering Stumble Upon. Oh my goodness is it addictive. These are a few of the sites I have particularly enjoyed so far:
jacksonpollock.org I could play on this site for hours. Use your mouse to make a Pollock style painting. That description doesn't do it justice, just try it... you won't be sorry!
jamesshuggins.com If you like you can call me sad but I LOVE cats and this had me giggling until I felt sick. Literally!
lookatabook.com This site is documenting a book which was posted between four artists in the USA and Ireland. Each artist designed a page and then sent it on. I've seen things like this before but I really like the style of this one, and the site is very interesting.