GL - George Lucas
HC - Hayden Christensen
RM - Rick McCallum
ILM - Industrial Light and Magic
LFL - Lucasfilm
PT - Prequel Trilogy
OT - Original Trilogy
SE - Special Edition
OOT - Original Original Trilogy
ANH - A New Hope
ESB/TESB - The Empire Strikes Back
ROTJ - Return of the Jedi
TPM - The Phantom Menace
AOTC - Attack of the Clones
ROTS - Revenge of the Sith
SW - Star Wars
IMO/IMHO - In my opinion/In my humble opinion
And a new one I figured out last week: 'ITA' means 'I totally agree'.
The Urban Dictionary is also really good when you get stuck.