A favourite for a while has been IMNSHO (In My Not So Humble Opinion)

RTFM, obviously, although I tell users the F is for friendly...
AFAIK
IIRC
and ALIAN (A Lawyer I Am Not), because it just looks so much better than the usual IANAL (I Am Not A Lawyer)
One that always, always confuses me is iow - people use it instead of writing In Other Words, but my first thought is always Isle Of Wight!