I went to the seed swap at the Berkeley Ecology Center last year, and it was a blast. Potluck, meeting other local gardeners and urban farmers, and a whole room full of free open-pollinated and heirloom seeds, many for varieties I'd never heard of.
Some of the seeds came from local seed libraries, while others were commercially grown packets that people hadn't gotten around to using, or didn't use up. This is one of my favorite things about both seed swaps and seed libraries. In a little urban garden, a packet of 30 seeds is often too many. You don't have room for thirty tomato plants, and certainly don't want that many of the same variety (unless you want your entire summer's harvest to ripen the same week).
Seed swaps and seed libraries let you take home just the seeds you need. Signs at the swap suggested 2-3 seeds for each plant you want, to allow for some not sprouting. This means you can easily try a lot of different varieties, without buying and wasting 90% of the seeds in each packet. After last year's seed swap, I grew four kinds of cucumbers, eight kinds of tomatoes, six kinds of peppers, and so on. It turns out that some of the results weren't quite what I'd hoped for, but that's okay: I had plenty of other ones to choose from.
(Sorry about the quality of the photos; I took them with my phone.)