Aunt Martha's Garden Heirloom Seed Company
Offers hundreds of varieties of heirloom seeds. I found it interesting that they include in their online catalog seeds for "seed saving" plant varieties like beans, cucumbers, eggplant and tomatoes.
Heirloom Seed Swap
Calling themselves the The Free Seed Exchange, HeirloomSeedSwap.com doesn't charge for you to trade your seeds, offer them for free or offer them for sale. The site is brought to you by the hosts of the Self-Sufficient Homestead Show, Johnny MAX and the Queen. They also have a Facebook page. BONUS: Go to the bottom of the page for a link to Jack Rowe's FREE online book Vegetable Seed Saving Handbook.
iVillage GardenWeb Seed Exchange
This seed exchange provides a place where users can offer seeds for trade and make requests for seeds they are seeking. Items for sale are not permitted. Please read the special instructions for the exchange and the more general Forum Instructions before signing up and posting. Note there are also plant exchange and seed saving forums at GardenWeb as well.
National Gardening Association
Visit their dedicated seed swap page using the link above, which is a free forum where you can find the seeds you've been looking for or share your excess seeds with others. Traditional swaps are a 2-way trade and the interested parties will contact you by phone or email. Even if you don't have any seeds to offer, you might still find someone willing to help you out with your request.
Seed Savers Exchange
Founded in 1975, Seed Savers Exchange is a membership organization. They publish the Seed Savers Yearbook that makes available to members the seeds of more than 13,876 unique varieties of vegetables, fruits, and grains. Each year members offer nearly twice as many vegetable varieties as are available from all of the mail-order seed catalogs in both the U.S. and Canada.
Here is a short video that explains a bit more about the Seed Savers Exchange:
The Old Farmer's Almanac Seed Swap
This is a forum at The Old Farmer's Almanac website set up specifically for seed swapping. If you've been searching for an elusive variety of vegetable or flower or have seeds to share, it's the place for you! To use the forum, you'll need to read and agree to the Seed Swap rules as well as register in order to post.