Painting with marbles: http://tinkerlab.com/spooky-marbled-spider-webs/
More colorful: http://www.letthechildrenplay.net/2010/07/super-sized-marble-paintings.html
Anyone remember the vintage box of bluing balls which looked like benningtons? (And I think it was listed under marbles on eBay.)
I tried to find a pic of that but all I got was a link to an old auction.