PSA = Public Service Announcement.
ETA = Edited To Add (or, sometimes, Estimated Time of Arrival).
YMMV = Your Mileage May Vary (as in your opinion may be different to mine so lets agree to differ).
IM((ns)H)O = In My ((not so) Humble Opinion.
IANAL/D = I Am Not A Lawyer/Doctor.
IME = In my Experience.
Any I missed that I might use and/or find useful?
ETA (from and inspired/reminded by the comments):
LOL = Laugh Out Loud.
ROFL(MAO) = Rolling On Floor Laughing (My Ass Off).