Taken from anime, where they usually have a bead of sweat, in a raindrop shape, running down the side of their head. Mostly used in embarassing, or strange, situations.
Depicts a nervous / anxious smile in ASCII art. The semicolon serves as a sweat drop (commonly seen in anime). Very frequently seen in Instant Messages or forums.
Woman: "Did you hear there was a big robbery right around the corner?"
Man in Ski Mask: "No... no can't say that I did. ^^;"