jargon.artrocity.com
New Hacker's Dictionary the Jargon File v 4.4.6
[common] A beast that randomly chooses one of a selection of signatures for appending to mail and news messages. The creature is most often mentioned directly when it has been in particularly good form and selected a signature appropriate to the topic being discussed; the construction “P.S.: good sigmonster, have a cookie” is not uncommon. While the are sigmonster programs floating around on the net, most hackers who keep one use a silly little Perl or Python script that they threw together in the middle of the night under the influence of far too much caffeine.