Recently I was invited to join the Great American Seed Swap on Facebook. I haven't signed up, but it looks like a group on Facebook for gardeners who swap seeds either one-on-one in a round robin style. I'm not associated with the group and you shouldn't consider this an endorsement of the group or anyone who trades seeds there. Don't have a Facebook account? You can always swap seeds at various gardening forums around the internet.
If you know of any other Facebook groups set up for seed trade leave a comment and I'll add them to this post.