Some of these are common in spoken language as well, but this is primarily a list of slang expressions used in chat rooms and text messages.
If you know of any more, please add them in the comments!
Trembling from fear, maybe the sound of teeth chattering (deu-deu-deu). [from 덜덜]
Yes / I understand / I agree / Shut up (ng ng)
Hi. Short for 하이.
This can be used as a middle finger.
Thanks (short for 감사합니다).
It's OK (short for 괸찮아).
Expression of anger, excitement, happiness, or any intense emotion. Kind of like "damn!", though it's not a bad word.
Teacher (short for 선생님).
(Used often by girls in particular)
Happy face. By far the most common Korean emoticon in existence.
ㅠㅠ or ㅠ.ㅠ
Sad / Crying face. Perhaps a close second to ^^. Korean girls are very dramatic.
This is an entire crying person, who is so distraught he's on his hands and knees.
This funny clip from 거침없이 하이킥 (High Kick) explains a few more in detail.