When sent by itself as a message via text, aim, mail, etc it has the same meaning as lol, which either has no meaning or means what you just said wasnt interesting. Pretty much if you get a ":)" that person has nothing to say and might even just not wanna talk to you.
bob: whats up
bob:same, except i just got home from the movies, the movie i saw was so funny, you gotta see it
joe: hey babe
joe: I had fun tonight
joe: you looked so pretty tonight i couldnt keep my eyes off of you
a way to indicate that you either want a blowjob or want to give someone one without actually saying the words " I want you to suck my dick" or " I want to suck your cock". This might be used to avoid awkwardness.
Joe: can i ask something?
joe: :0 <==8
kim: ;) sure babe,