Remember that the most cost-effective and energy efficient use for leftover paint is reuse. If you do have paint left at the end of a project, apply another coat to an area which could use extra protection. Mix smaller quantities of leftover latex paint together for use as a primer coat on a larger project. Be sure to keep a small amount of paint on hand for touchups.
If you can't reuse your leftover paint, you probably know someone who can. Offer leftovers to a friend, neighbour, or your local community centre, church, or theatre group.