Used in chat programs and SMS's to infer sarcasm prior to that point.
Can sometimes be accompanied by a <s> at the beginning of the sarcastic phrase.
This is particularly useful for those who don't seem to understand sarcasm in written form, and can assist in avoiding lengthy discussions based on misunderstandings.
<s>Wow, that story was really interesting</s>
Person 1: Work is great today!
Person 2: </s>?