This entry is a Bike Hacks Classic.
Here are plans for an excellent low cost bike trailer. The plans are free to anyone that asks for them, but the organization distributing plans takes their mission very seriously and asks for a little something in return. They ask that you:
1. Tell them what you are using the bicycle trailer for
2. Send them photos of the bicycle trailer you build
3. Tell them how to improve the instructions or the trailer
4. Accept that they might post your email on their blog
Sounds like a pretty good deal to me. I think this is a fantastic idea. People all around the world can put low cost trailers like these to good use, building self-sufficiency without fossil fuels.
Photo from carryfreedom.com