Indicates the presence of sarcasm.
Man, I wish George Bush could take another 4 years, especially after his work on torture- who'd have thought such a fair and moral decision could have been made?
The /S is known as the sarcasm switch. When you are typing a post use it at the end of your post so people know you are actually being sarcastic.
One good thing about you being wrong is the joy it brings to others. /S
It may be that your sole purpose in life is simply to serve as a warning to others. /S