MyCityGardens is a group that aims to connect people with a zeal for gardening but no yards with people with yards who'd love to see them cultivated but just can't do it themselves.
Their Web site features a map showing where people are looking for plots to garden and where people have plots that can be gardened.
It is true that so far, there are more gardeners than landholders in our system. BUT there is more gardening space out there than is visible on our map. We've observed the people with land who sign up to yard share tend to stay 'hidden' in our database (i.e. don't have a public profile), but reach out to gardeners with public profiles. So gardeners, create a post, even if you don't see available land nearby.