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AFAIK

"As Far As I Know" - Stating that what you posted is to the best of your knowledge.

FTW

"For The Win" - Generally appears when someone thinks that something is the best and will win.

IMHO

"In My Humble Opinion" - Makes it clear that you were stating your opinion.

OP

"Original Poster" - The person who originated the discussion.

PM

"Private Message" - Most forums support a sort of internal email system which allows members to send messages to each other.

QFT

"Quoted For Truth" - Often used when quoting another post and stating that you think their statement is very true and worth repeating

Slashdot Effect

Also "Slashdotting" or "Slashdotted" - When a news item is posted on the popular website Slashdot the resulting flood of website visitors to the page that the news was about can slow down or even stop a website altogether.

YMMV

"Your Mileage May Vary" - Most often seen when referring to something that may or may not happen as described.

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The above terms are generally most often used in forums. There are many others that have crossed over from chat rooms, instant messaging, email and newsgroups. For a more complete list of those visit Wikipedia or the Urban Dictionary as the full list of internet acronyms and terms would take up a book.