ETA

Eta (letter), of the Greek alphabet (Η, η)

Abbreviations:

ETA (Euskadi Ta Askatasuna), a Basque separatist organisation
E.T.A., an independent film currently in production
ETA (band), a Swedish hardcore punk band
ETA movement, a watch movement
ETA programming language
ETA Systems, defunct manufacturer of ETA10 supercomputers
Electronic Technician's Association
Electronic Travel Authority, Australian electronic visa
Environmental Transport Association, a UK motor car breakdown and rescue organisation
Estimated Time of Arrival
Estonian Broadcasting Agency (Eesti Teadete Agentuur)
Ethiopian Teachers' Association, a trade union centre for Ethiopia


In other words, ETA is not equal to PS (Post-Script) or Edit.

If you're (generic you) are adding something to your post, don't write ETA. Your post already arrived, so did the edit. It doesn't have an ETA.

/rant

EDIT: Above material was quoted from Wikipedia.

EDIT 2: I've been explained thoroughly that ETA is "Edited To Add", which is a WTF to me... never ever ever heard it, and it still drives me a bit nuts that they (that universal they meaning whoever the eff invented it) coined that phrase/abbreviation that already has another, wildly known meaning.

I'll stick to my "EDIT".