Got a DIY project you found on, well, some site or another, or got a small area you want to try out a paint colour on? The Wise Bread blog says that a stash of powdered milk—which they list more than a dozen other clever uses for—can be used to mix up a reasonable simulation of the stuff you pay a good amount for at the hardware store:
Mix 1 part water to 3 parts powdered milk until you get something around the consistency of paint. Blend in a water-based colour if you don't want neutral white, and paint your heart out. As with normal paint, let it dry thoroughly between coats (ie: 24 hours).
I haven't tried this myself, but the stuff certainly gets pretty paint/glue-like in the right proportions. Hit the link below for more tricky uses of the white stuff.