Quite a few times, I've seen people use the acronym(?) 'ETA' to imply that something has been edited, but for this life of my I can't imagine what it stands for. 'ETA' to me is 'Estimated Time of Arrival'. Wikipedia and dictionary.com don't mention anything about it being used in this fashion as an edit marker.
Are people using it mistakenly? The most sense I can make out of it myself is that it's substituting for the greek letter Η,η (like N.B. is Latin, or somesuch), but why wouldn't people just use 'E:' or even 'H:' in this respect?
EDIT: lilacsigil suggests it's "Edited to Add". I'd thought that might be the case, but I assumed it was a "backronym" of sorts, a phrase that was made up to justify the use of 'ETA' in the first place. The only reference I can find for it is:
... which appears to be user submitted, and contains such gems as "EtA: Eternal Allegiance (fan site)" and "Evangelical Training Association".
I may be entirely mistaken, and this might be an established convention, but I can't find discussion on it anywhere, and it's one of those things that always makes me twitch when I see it.
EDIT: Well apparently, it's Edited to Add, and it's popular in the blogosphere. The Wiki page for "ETA" has since been edited to include it, so consider me satisfied.