Having one eye closed. This can be for any reason.
If the eye is closed involuntarily, as a result of surprise, pain, or embarrassment, it is often a wince as well. If the tongue is sticking out, it is also ;p. If the character is grinning, it is also ;D. If the closed eye is drawn like one side of >_<, it is also >_o.
source: danbooru wiki