I wouldn't have the nerve to seriously slip one to a stranger, but my goodness there have been times where I wish I could have, and I've also had times where I've wished someone would tell me to hush. If you're interested in purchasing some, check out Set Editions. Another set of theirs that I particularly enjoy is the "Mercy" set.
There have been many times in the last 4 months where I should have given L the one that says, "I am in a bad mood and taking it out on you" as soon as he walked in the door after work (work is not the most fun these days), but alas, I didn't know about these! Just having a card to give him that said that and writing, "I'm sorry," on the inside would automatically make me laugh and make the whole situation better. The "Truth Hurts" cards also make me laugh, but mostly from the perspective of getting any of them when I wasn't expecting it. Anyone have any fun card vendors they like??