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A .sig is template that many fans add after their messages on mailing lists.
The sig can contain the fan's name, their fannish affiliations, political and personal statements, in-jokes and shout-outs, and quotes. It was also a place to pimp their own websites and other fanworks.
A possible misuse of a sig? Plumping: "Are you just interested in having another club name to put in your .sig, or are you interested in participating in an 'email club' which searches out/shares/discusses anything and everything about TGBO (The Great Bald One)?" 
Also: "Many people pay for either time on-line or the amount of information they download. Full screen pictures of the Liberator, 18 character high pictures of your name, endless cute quotes, etc. cost people money. They also mark you as someone with an inflated sense of his own importance. Try to keep sig files to 7 lines at most, preferably 4 or 5, and never use a sig file that is longer than the text of your message."