Check with your local municipality for the specific rules in your area.
Most municipalities allow dried latex paint to be disposed of in the regular trash pick up. Wet paint must be completely dried first. Oil-based and other non-water-based paints must be disposed of through the local hazardous waste process. Some cities and towns offer an annual drop-off at your center.
There are also waste paint hardeners that can be purchased. They are packets that you add to wet paint. It speeds the drying process and are fantastic when you have more than a little paint left in the can.
You may also be able to donate paint if you have enough to paint one or more rooms.