If the paint has dried and hardened, this can be safely disposed of with other household solid waste (waste haulers will not take liquids). Paint can be dried with sand, cat litter, sawdust, by leaving the paint can lid off, or by pouring the paint onto paper. The paint does not have to be hard, just not in a liquid form.
Large quantities of paint in usable condition could be donated to a local school or non-profit organization. Large quantities of unusable paint can be taken to Environmental Enterprises (800) 850-3587, and disposed of for a fee.