:(

The converse of :), an emoticon often used do convey disgust or general unhappiness--sometimes in association with error 404s encountered while browsing the Interweb. Within more recent times, :( has been expanded to include the emotional state just boredom and noding for reasons unbeknownst to most people (including myself).

:( is unique because, like :), it creates some confusion when used within parenthesis, even though it doesn't create a double-character as :) does.

Here is an example of :( being used within parenthesis:
blah blah (thing that disappoints me :()

It's impossible without context clues to tell if this is a frowning emoticon or simply a person in serious need of plastic surgery; thus, some people have begun using :< more actively than :(. (It's also easier to type).