Accuweather shared this great photographs of mammatus clouds taken by its Facebook fans. The clouds are also known as mammatocumulus clouds. The name comes from the Latin word "mamma," which means breast or udder. The Wikipedia entry for Mammatus clouds also has another gallery of photographs. NOAA says Mammatus clouds most often "are seen hanging from the anvil of a severe thunderstorm, but do not produce severe weather."
The clouds do not mean a tornado is about the form as they are usually only seen after the most damaging part of a thunderstorm has passed. Take a look: