WinterSown.Org offers free tomato seeds and free variety seed packs, mostly flowers, via self addressed stamped envelopes along with growing tips and tricks.
FreeFlowerSeed.com will send you a packet of seeds for SASE as well. This year, they gave me some Sweet Peas, just like my grandmother used to grow on her fence!
Ed Hume Seeds offers free seeds for their "Plant A Row For The Hungry" program. Last year they gave me carrots. I haven't received this years packet, but that won't stop me from donating my extras to our local shelters.
Burt's Bees used to offer free seeds via an electronic form on their site, but due to popularity has run out of free seeds. I feel happy and partial responsible for this, due to my posting their link on wikiAnswers regarding free seeds. I did my part to fight Colony Collapse Disorder last year and I'll increase my efforts this year. They sent me a nice packet of wildflowers while they still had some left.
Biblical Gardens also offers free seeds to anyone willing to send them a letter. The varieties are based on those plants that appear in the bible, of course, however I've never received anything other than Shasta Daisies from them as their supplies seem highly limited.
GardenWeb has an excellent seed trading forum which frequently has SASE seed offers along with rare varieties of plants for trade.
Dave's Garden is another excellent forum for seed trading, growing information and occasional liquidation sales from name brand companies. It's nice to pick up a few seeds for next to nothing when a company is simply trying to clear out old stock, eh?
Last but not least, you could always join the Seed Savers Exchange. Membership isn't required to order from their website or from the catalog they send out for free, but it is to gain access to their seed trading album. Personally, paying a membership fee to gain access to anything "free" isn't worth it. The purchasing catalog is great though, with an excellent variety of heirloom and certified organic seeds or transplants.
Obviously, these are not all the resources out there, just the ones I've personally used and confirmed to be authentic. A note however; most of these people are giving seeds away from their own gardens or from donations, try to send them some of your own saved seeds at the end of the season. Let's keep the cycle going and make sure everyone will have enough seeds next year!