Some of the best art supplies come from the trash or recycle bin! Give your kids some duct tape, screws, nuts, glue - whatever and let them have at the trash/recycle pile!
Kandinsky art by No Time for Flashcards
Love the color wheel from Little Illuminations
Robots from Spoonful
Some fun recycled art from maureencrosbie on Flickr
and I love Modern Parents Messy Kids recycled art roundup