HelpX (Help Exchange) is "an online listing of host organic farms, non-organic farms, farmstays, homestays, ranches, lodges, B&Bs, backpackers hostels and even sailing boats who invite volunteer helpers to stay with them short-term in exchange for food and accommodation."
These are the HelpX experiences I've had:
Válor, Spain (Granada)
December 22, 2011 - January 7, 2012
March 2017