^_^; - A chronically overused emoticon by most any otaku who's ever used IRC or any other chat server. It's the typical anime sweatdrop, which is used to indicate embarrassment.
Often when someone professes they "love hentai," replying with a quick "^_^;" means "Eeew! (me too, but i won't admit it)". It's a really hard habit to break once you start using it.
Another variation is ^^; or the emphatic ^_^;; (TWO sweatdrops, ooo!).
Personally, I prefer o_O (often used by people trying to break the now-trite ^_^; habit).
See also ^_^.