One free site is InterPals Penpals - Make friends online and find free pen pals from around the world!
I have been using it for years, for snail mail pen pals, language practice and also meeting local people. It is huge community and you can be sure to receive lots of junk mail as well as real mail. If you feel like some certain countries are on your "not interested" list, you can block them, also you can make sure only people of certain age, sex or location can write to you.