Hundreds of emoticons have been proposed, but only a few are in common use. These include:
:-)
:-(
;-)
- `half-smiley' (ha ha only serious);
also known as `semi-smiley' or `winkey face'.
:-/
- `wry face'
The first two listed are by far the most frequently encountered. Hyphenless forms of them are common on CompuServe, GEnie, and BIX; see also bixie. On USENET, `smiley' is often used as a generic term synonymous with emoticon, as well as specifically for the happy-face emoticon.
It appears that the emoticon was invented by one Scott Fahlman on the CMU bboard systems around 1980. He later wrote: "I wish I had saved the original post, or at least recorded the date for posterity, but I had no idea that I was starting something that would soon pollute all the world's communication channels." [GLS confirms that he remembers this original posting].
Note for the newbie: Overuse of the smiley is a mark of loserhood! More than one per paragraph is a fairly sure sign that you've gone over the line.