I stumbled across the Hope Revolution the other day. The premise is simple: write a note and leave it somewhere to brighten someone else's day. Doesn't have to be fancy. In fact, my favorite from the Flickr pool is a couple of hand-written lines on a napkin.
City litter ordinance issues aside, I like the concept. Chalk on the sidewalk, a note under your pillow, a surprise in the mailbox--who doesn't love? (Some of these are more appropriate between people who know each other, of course. Please don't leave unsolicited mail under a stranger's pillow. That would be creepy. In a bad way.)
Also, check out Mysterious Letters. Ambitious and inspiring.