Take oil-based paints and lead-based paints to a hazardous waste drop-off facility. Latex paint is not hazardous and can be disposed of with normal trash once it has solidified.
Search by zip code on Earth911.com to find a local hazardous waste drop-off facility. To harden latex paint, mix kitty litter, sawdust, shredded paper or paint hardener with the leftover paint. When it has hardened, throw it away uncovered. Alternatively, scoop the hardened paint out of the can, discard the hardened paint and recycle the empty can.
Some agencies, such as Metro Waste Authority, charge a fee for taking latex paint.