OMG - most say "Oh my God", mine is for "Oh my Goodness"
NTB - Not too bright
BTW - By the Way
ROTFLOL -Rolling on the Floor laughing out loud
J/K - Just Kidding
IMO - In my opinion
Some I might start using:
TRDMC - Tears Running Down My Cheeks
ONNTA - Oh No, Not This Again
PITB - Pain in the behind
http://www.magicpub.com/netprimer/acronyms.html