A contronym or antagonym is a word that is its own antonym, that is, a word that generates two opposite meanings, such as fix, which means to restore or to castrate. A contronym may be the result of a historical process whereby a word's meaning shifts so radically that it reverses. This is known as semantic reversal, and a good example is manufacture, which originally meant to make by hand but now means to make by machine. In the case of a contronym, the new meaning does not replace the old one; rather, the two co-exist. A contronym may also occur when two separate words with opposite meanings become pronounced in the same way. An example is the word cleave, which means to adhere and also to separate; the original Old English words were cleofian and cle-ofan respectively. The A.Word.A.Day mailing which introduced this word into my vocabulary notes that contronyms are also called Janus-faced words, after the Roman god Janus who had two faces that looked in opposite directions.

Fun with Words at http://rinkworks.com/words/contronyms.shtml has a long list of contronyms, some hightlights of which are