:-) Your basic smilie. This smilie is used to inflect a sarcastic or joking statement since we can't hear voice inflection over Unix.
;-) Winky smilie. User just made a flirtatious and/or sarcastic remark. More of a "don't hit me for what I just said" smilie.
:-( Frowning smilie. User did not like that last statement or is upset or depressed about something.
:-I Indifferent smilie. Better than a Frowning smilie but not quite as good as a happy smilie.
:-> User just made a really biting sarcastic remark. Worse than a :-)
>:-> User just made a really devilish remark.
>;-> Winky and devil combined. A very lewd remark was just made.

Those are the basic ones...Here are some somewhat less common ones:

Changing the mouth:
:-[ A vampire.
:-E Buck-toothed vampire.
:-F Buck-toothed vampire with one tooth missing.
:-7 Just made a wry statement.
:-* Just ate something sour.
:-@ Screaming.
:-# Wears braces.
:-& Tongue-tied.
:-Q Smoking.
:-? Smoking a pipe.
:-P Sticking out tongue.
:-S Just made an incoherent statement.
:-D Laughing out loud.
:-X Lips are sealed.
:-C Really bummed.
:-/ Skeptical.
:-o Uh-oh.
:-9 Licking his/her lips.
:-0 No yelling! (quiet hours in the dorm) - also an orator.
:-` Spitting (tobacco).
:-1 Normal.
:-! Normal.
:-$ Mouth wired shut.
:-% Banker.
:-q Trying to touch tongue to nose.
:-a Same as above on other side.
:-e Disappointed.
:-t Cross.
]:-i Semi-smiley.
:-] Blockhead.
:-[ Unsmiley blockhead.
:-} Chapped lips, or bearded.
:-{ Indifferent with moustache.
:-j Left smiley.
:-d Left smiley razzing reader.
:-k Beats me.
:-\ Undecided.
:-| Have a day.
:-< Real sad.
:-x Lips are sealed.
:-c Bummed out.
:-v Talking head.
:-b Sticking out tongue.

Changing the nose:
:*) Drunk.
:^) Has a broken nose.
:v) Broken nose in the other direction.
:_) Nose is sliding off his/her face.
:<) From an Ivy League school.
:=) Two noses.
:o) Clown.
:u) Funny looking nose (left).
:n) Funny looking nose (right).

Changing the eyes:
%-) Staring at the screen for 15 hours straight.
8-) Wearing sunglasses.
B-) Wearing glasses (could be horn-rims).
O-) Megaton, cyclops, or wearing a diving mask.
.-) One-eyed.
,-) Winking, one-eyed.
g-) Ponce-nez glasses.

Changing more than one character:
|-I Sleeping.
|-O Yawning.
%-6 Brain dead.
|^o Snoring.
:,( Crying, but without a nose.
8-| Suspenseful.

Adding extra characters:
::-) Wears glasses (four eyes).
B:-) Glasses on top of head.
8:-) Little girl.
:-)-8 Big girl.
:-{) Has moustache.
:-#) Bushy moustache.
{:-) Wears a toupee.
}:-) Toupee in an updraft.
:-)~ Drooling.
:-~) Has a cold.
:'-) So happy, he/she is crying.
=:-) Hosehead.
-:-) Punk rocker.
+-:-) The Pope.
`:-) Shaved off one eyebrow.
,:-) Shaved off the other eyebrow.
O :-) Angel.
C=:-) Chef.
*<:-) Wearing a Santa Claus hat.
E-:-) Ham radio operator.
8 :-) Wizard.
@:-) Wearing a turban.
[:-) Listening to a walkman.
:-)) Double chin.
>:-) Devil.
(:-) Big smile.
+:-) Priest.
:-)X Wearing a bow tie.

Misc:
(-: Left handed.
[:] Robot.
:-{} Wearing lipstick.
:'-( Crying.
<|-) Chinese.
<|-( Chinese, but doesn't like these kind of jokes.
-:-( Real punk rockers don't smile.
@= Pro-nuclear war.
(8-o Mr. Bill. (From original Saturday Night Live)
*:o) Bozo the Clown.
3:] Pet smile.
3:[ Mean pet smile.
d8= Beaver with goggles and hard hat.
(:I Egghead.
<:-I Dunce.
K:P Kid with propeller beanie.
:-: Mutant.
Invisible.
X-( Dead.
[] Hugs.
~~:-( Getting flamed.
O |-) Religious.
8 :-I Unix wizard.