Most homeowners have a stack of old paint in the basement or garage. In most places it's illegal to simply discard old paint cans in your trash. You can drive them to your local landfill and drop them off with all the other hazardous waste, but that's time-consuming and a nuisance. Luckily, there are many other options, without being harmful to the environment or breaking the law.
As long as your old paint is tightly sealed and stored in a cool place, it is neither flammable nor toxic. Store paint upside down and it will stay fresh and usable for years. Wet paint of any type should never be poured down the drain or discarded with your regular trash pickup. Not only is this ecologically unwise, it is illegal in many areas.
Drop Off or Recycle Old Paint
Most counties have a Household Hazardous Waste (HHW) program, allowing residents to drop off old paint along with used motor oil and household chemicals for safe disposal or recycling. The HHW drop-offs usually take place twice a year. Check the link in Resources to find your local disposal dates and locations.
Put it to the Curb
Most waste disposal companies will take opened, dried-out cans of paint. This means that you can simply put cans of dried paint out with your regular trash. Leave cans of paint unopened in a dry location. Depending how much paint is in the can, it can take days, or even weeks, for the paint to harden. To speed the process, stir in some absorbent cat litter.
Donate Old Paint
Check with churches or charitable organizations like Habitat for Humanity. If you have usable paint, they may be happy to take it off your hands. Find private individuals who might appreciate free paint for home or craft uses through online bulletin boards like craigslist.org and freecycle.org.
Paint dating back to 1978 or earlier probably contains harmful lead and should not be opened or used. See References for how to deal with lead paint. Most solvent-based paint, and paint in spray cans, is considered hazardous waste and must be disposed of in accordance to local and federal laws. Partially used spray paint cans are flammable and must be kept away from direct sunlight or other heat sources. They contain propellants, which, when overheated, can cause the can to explode.