Muffin tin paint palette
When your child wants to release his or her inner Jackson Pollack, here's an easy way to keep the paints under control: pour one paint color into each cup in a muffin tin. If you don't fill all the spaces, fill the remaining ones with water and/or leave a couple empty for mixing colors.
It works well because the cups are large enough to hold a fair amount of paint, and the muffin tin itself is stable enough that it's (almost) impossible to knock over.
More: Paint hacks