The string "-- " (That's two dashes and a space) on a line by its own, that goes between the message body and the signature in Usenet posts. Helps the newsreader strip out the signature in replies. Some also display the .sig in a different colour.

Some pedantic people throw a wobbly if you don't include it or (heaven forbid) don't put in the space. Most newsreaders include this, but one or two (not mentioning any Microsoft products in particular) don't.