My City Gardens lets homeowners with extra yard space connect with others in their community who want to have space to garden. This is great for urban folks who want to grow their own food, but live in apartments. It's also a great way to meet your neighbors.
The site has a searchable database of both available garden space, and people who want to garden. Right now, it's only for Mass. residents-- but seems like it could easily be recreated for other states.