Goodwill or the Salvation Army are good places to donate your old coats.
Another wonderful organization would be happy to take your old or outgrown coats is www.onewarmcoat.org/donatecoat.php. This charity helps the homeless and other people in need keep warm during the winter. This is especially now important since the tough economy has impacted many families across the country.
There are two ways you can participate. You can type in your zip code to find a coat drive near you. I tried a couple of different zip codes and it seems that bigger towns/cities have drives that are closer than if you live in a smaller town.
Another way to participate is run a coat drive yourself. The website provides detailed instructions on how to coordinate one in your town.