In the "Puppies Wearing Hats Eating Bacon Sharing Facts" feature on Mental Floss this week, one adorable dog explains, "Vinegar Valentines, popular in the mid-1800s, were cards that insulted the recipients, insinuating that they were lazy, promiscuous, or prudish. Postmasters sometimes refused to deliver them." Wikipedia offers woefully little information about them, but the Royal Pavilion and Brighton Museum's photostream on Flickr offers a few good examples.
The above is a card from 1906 by Raphael Tuck. And if you don't want to insult a lady cardshark, you can also direct your venom at sugar daddies, smokers, tailors, and bookworms.