Offer up leftover supplies from knitting, painting, and sewing projects through Made4Aid.org, which sells them to artists and crafters and donates proceeds to Doctors Without Borders.
Overrun with Crayolas too stubby to color with? Ship them off to Crazy Crayons, which recycles unusable crayons into brand-new ones.
Creative reuse centers collect unexpected art supplies—like old VHS tapes, cardboard tubing, wire hangers, paper scraps—and distribute them to artists. The centers are popping up across the country: the East Bay Depot for Creative Reuse in Oakland, California; the Scrap Exchange in Durham, North Carolina; and Trash to Treasure in Fort Lauderdale, Florida, are just a few examples. (Google "creative reuse center" to find one near you.)