Some day, after a thunderstorm, you may look up and gasp, "My word, what in blazes is that!? The sky is filled with strange clouds, bubbly and bulbous like popcorn but spread across the sky like stratus clouds. Goodness, it's as if the heavens are boiling!"
Don't worry, it isn't the apocalypse. What you are actually seeing are mammatus clouds, and contrary to popular belief, they're nothing to worry about. Mammatus take their name from the Latin word for breast, mamma, because of their resemblance to cows' udders.
Mammatus clouds occur as a side-effect of the anvil appearance of a thunderhead. Unlike most clouds, mammatus form in sinking air, instead of rising air; this results in their odd appearance. As updrafts carry moist air to the top of a cloud, it spreads out and loses its upward momentum. The air, heavier than its surroundings, begins to fall back down, heating up as it does. This warming evaporates the water contained in the air. However, if the energy requirement to evaporate the water droplets is greater than the amount generated by them falling, the droplets will become cooler than the surrounding cloud, and appear below the cloud as mammatus formations. The life of these formations depends on their composition; large droplets of water or ice crystals will result in longer-lasting mammatus, as they take longer to evaporate.
Mammatus are fairly common, but not typically very long-lived, so it's not surprising that many people aren't aware of them and are startled upon catching their first glimpse of the odd and sometimes ominous-looking clouds. It is a common misconception that mammatus appear right before a tornado forms, but in fact the two phenomena are totally separate and unrelated (other than the fact that they can both result from severe thunderstorms). I personally believe they're one of the coolest sights in nature, particularly when sunlight reflects off of them, highlighting their curves and valleys.
You can catch some great photographs of mammatus clouds at http://www.targetarea.net/photomam.html.
Sources:
The UIUC Department of Atmospheric Science's WW2010 pages, http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/cldtyp/oth/mm.rxml.
http://www.tapestryweb.org/tornado/mammatus.html
http://www.crh.noaa.gov/lot/photos/Jul2004_mammatus/july13.htm (These are the ones I saw!)
And, as is often the case when I node, personal experience (as noted above).