O_O
When online, used to express a state of utter wtfness, or when you simply cannot believe the stupidity of what the person above has just said. It kind of reminds me of a paralysed frog.
Example:
Person: OMGWTFBBQ, lawlorz i sooooooooooooooooo pwned
joo /\/00b!!!1 1!1!1!!1im sooo awsum!!111!11one!!! !11!!!!eleven!!! "11!"211! LOLOOLOLOLOLOLOLOLOLOL LOLOLLOLOLLOLIPOP
You: O_O
Person: OMGWTFBBQ, lawlorz i sooooooooooooooooo pwned
joo /\/00b!!!1 1!1!1!!1im sooo awsum!!111!11one!!! !11!!!!eleven!!! "11!"211! LOLOOLOLOLOLOLOLOLOLOL LOLOLLOLOLLOLIPOP
You: O_O