Section punctuation mark, typed by pressing and holding the ALT key down while typing the numbers 167. "§" is used to denote sections in legal documents. It is also used at the end of sentences to denote sarcasm in written electronic media such as Facebook or Livejournal status updates and resulting comments in order to avoid confusion as to what is a sarcastic remark, and what is not. Invented to replace having to type "I'm being sarcastic, by the way".
Oh yes, I find you very smart.§
Are you a genius?§
I'm thrilled I have to stay at work after 5 PM on Friday.§
Yes! Let's all go jump off the cliff!§