I started doing these self-promo cards in '99 using a polaroid theme. Since then I've done a variety of printings, with a number of different images for different purposes. Sometimes they go out in sets, sometimes they get handed out individually.
What's interesting is how many people actually think they're polaroids at first.

Most importantly is that people hold onto them. I've seen a lot of them on desks and bulletin boards of people I've sent them to, which is a great thing!