Emoticons are especially useful in the textual worlds of chat rooms and e-mail, where people can't see the facial expressions you make as you communicate. A quick :-) is a cute way to show that you're smiling as you type. Don't use emoticons as a replacement for more complex forms of communication, however. They serve as punctuation marks, not whole statements.
Here's a small compilation of emoticons I've seen or made over the years. (By the way, I usually prefer to use the equal sign - close parentheses smiley = ) rather than the conventional :-). I feel that the equal sign is cuter, and more expressive.)
= ) smiley
= ( saddey
>= ( frowney
; ) winkey
= O gapey
= D laughey
= P tongue-outey
= b... drooley
- ) cyclops
8 ) weird glasses
=^) big nose
&= ) curly hair
= { moustache
= / lopsided mouth
>= I Vulcan (don't know how to throw in pointy ears, though.)
| ) sleeping
= * puckered
More to come!