My favorite thing to do! Seed snatching as well as knocking at peoples' door to ask for a cutting, what a treat! And by doing that I've made a good number of friends. Plant exchange among friends, it is also another wondeful past time.
I'm always glad to "meet" another seed snatcher. And I think I'm too hesitant sometimes to ask for cuttings... you're right, it can be a great way to meet people! (I mean, the worst that can happen would be that they'd say "no"... it's not like they'll sic the dog on you for asking, LOL.)
A lot of my garden comes from "passalong" plants and shared seeds, and I agree that it's wonderful. Having a "history" like that makes the plants just that much more special to me.