Worldwide Opportunities on Organic Farms (WWOOF) started as Working Weekends on Organic Farms in England in 1971. Since then, it has grown to at least 99 countries around the globe, however only 50 actually have registered, national databases. WWOOFing brings together both hosts and volunteers who have at least one thing in common: the desire to farm in a sustainable, organic manner.
This is how it works (although it may differ from country to country): You sign up on the corresponding WWOOF organization in the country of which you either live in and are traveling to. Organizations may require a nominal registration fee (completely worth it in my opinion). If you have a farm and would like volunteers to help you on your farm, you set up a profile designating exactly what it is that you farm and where you are located. If you are looking to volunteer, you can peruse the various host listings and see who is looking for help at specific times of the year and in which part of the country the host is located.
I had originally planned to volunteer all across Japan, working my way up from Okinawa all the way to Hokkiado, the northern most island of Japan. Unfortunately, the earthquake and the subsequent, horrific tsunami that devastated the nation altered my plans. I would have loved to help in the aftermath clean-up directive, but I was a little afraid of turning green. However, I was able to volunteer for one WWOOFing experience in Yomitan, Okinawa. This is where I spent two of the best weeks of my entire life.