World Wide Opportunities on Organic Farms (sometimes Willing Workers on Organic Farms) is a grassroots organization that brings together organic farmers and farmhands. They have a small yearly membership fee in many countries, which covers the cost and continued production of the WWOOF handbook & listing for the area. Stays can last from several days to months (or even years) on end. WWOOFing and hitchhiking can compliment each other nicely.
from their website:
"WWOOF is a world wide network - It started in the UK in 1971 and has since become an international movement that is helping people share more sustainable ways of living.
WWOOF is an exchange - In return for volunteer help, WWOOF hosts offer food, accommodation and opportunities to learn about organic lifestyles.
WWOOF organisations link people who want to volunteer on organic farms or smallholdings with people who are looking for volunteer help."