This came up in a post of morriganslayde's where she was talking about how as an American in Australia she was sick of being called a Yank all the time (which I only just found out, some take offence to). Along with Yank, there are a few other words that Americans get called (Seppo, Merkin) but I've realised I have no idea what Australians are called overseas, only what we use to describe ourselves!
Come on, take your best shot! I promise not to get mad!
EDIT: For those of you wondering, Seppo is short for Septic Tank, which is rhyming slang for Yank. The term came about during the Second World War and it kinda stuck. As for Merkin, well, a Merkin is a pubic wig. If you're wondering why people from the US get called that, just say the word American as quickly as you can. Enough said.