I think the most unintentionally effective thing like this I've ever done was, in high school, hand-writing a short paragraph about anarchism (in a typically CrimethInc-ish fashion) with a link to CrimethInc on it, folding it up, writing "OPEN ME" on the front, and leaving it in a boys' bathroom stall.
A person found it, read it, went to CrimethInc, read a lot of their material, got pretty into anarchism, and - by chance - became a friend of mine. It wasn't until at least a year or two later that I casually mentioned the little secret message and that person incredulously told me that they were the one who found it.
(I'll have to think a bit about what I would do if I were doing something like that right now. Maybe I should revisit the hand-written note concept, huh?)