i nominate my old sig, my countdown to the end of everything which had a beginning. that got lots of people talking (most of you are too young to remember)
Sigs - educate and inform - just like the BBC's original mission statement. (ha!)
Useful b/c they tell the reader an enormous amount about the reader, convey their passions - I've discovered a huge amount as a result of quotations in people's sig-files. (Follow the quotation to its source - discover new ideas, theories etc you'd never have encountered otherwise. Unless of course you're already omniscient.).
Useful in a 'student' forum, I should imagine. [/Stream of Consciousness]