Worldpackers (Volunteer in Trade for Accommodation)
Worldpackers connects travellers looking to exchange their skills for accommodation by matching volunteers and hostels around the world, and offers placements with internationally recognized NGOs and non-profits in more than 140 countries. The website is free to join/make an account on initially (you can sign in with Facebook) but once you find a host you’re interested in messaging, the fee is $49 for the year.
“This is my go-to now, and provides a wide range of volunteer opportunities where you can volunteer in trade for free accommodation.” — recommended by Nora
To learn more: https://www.worldpackers.com/