Here are some of the most common acronyms found in newsgroups (and in some e-mail)

AFAIK	- As far as I know 
BTW	- By the way 
EOM	- End of message 
HTH	- Hope this helps 
IMO	- In my opinion 
IMHO	- In my humble opinion 
IMNSHO	- In my not so humble opinion 
IOW	- In other words 
LOL	- Laughing out loud (some people seem to use this for "lots of luck") 
OIC	- Oh, I see 
ROTFL	- Rolling on the floor laughing 
TIA	- Thanks in advance 
TTFN	- Ta ta for now 
YMMV	- Your mileage may vary

Emoticons

:-)	Smile 
:-D 	Laughing 
;-) 	Winking 
:-/ 	Scowling 
:-O 	Surprise 
>:( 	Angry 
:'-( 	Crying 
:-p 	Sticking out the tongue