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Heteronyms are words that can be pronounced two different ways, and the pronunciation changes the meaning of the word. The name comes up, I think, because in a sense they are two distinct words that happen to be written in the same way, hence the Greek roots "homo" and "graph" meaning "same" and "writing."
I thought this up on a road trip with a friend and we spent a half hour or so trying to think of examples. Since then I found an exhaustive list based on grepping the OED which removed whatever fun was in that.
Nevertheless, here is a simpler list of the words that I think are the most interesting examples in that they represent words with absolutely no semantic relation, unlike words like "house" or "extract" with related meanings.