Buy Nothing: The best known of the free exchange sites started in 2013. Buy Nothing primarily focuses on their new app, but also exists in location-based Facebook groups.
Freeya: This app shows listings based on geolocation and handles many logistical parts of coordinating pickups -- including contractin the next person if the first person doesn't show.
Freecycle: This site, founded in 2003, boasts nine million members in 5000 towns worldwide. If your town doesn't already have a Freecycle group, you can even start one.