I'm not at all fond of Internet slang/terms, so I try to avoid them. These are the ones I do use on occasion:

• AFK - Away from keyboard
• AKA - Also known as
• BB - Babe (?)
• BBL - Be back later
• BFF - Best friends forever
• BRB - Be right back
• BS - Bullshit
• BSoD - Blue screen of death
• BTW - By the way
• ELNS - Eller noe sånn (Norwegian for "something like that")
• FAQ - Frequently asked questions
• FGF - Fucking good friends
• FFS - For fucks sake
• FML - Fuck my life
• FTW - For the win
• FYI - For your information
• GL - Good luck
• HF - Have fun
• IMMD - It made my day
• IMO / IMHO - In my (honest) opinion
• IRL - In real life
• JS - Just saying
• LMAO - Laughing my ass off
• LOL - Laughing out loud
• NP - No problem
• NSFW - Not safe for work
• OMG - Oh my god
• PEBKAC - Problem exists between keyboard and chair
• RIP - Rest in peace
• ROFL - Rolling on the floor laughing
• SOB - Son of a bitch
• STFU - Shut the fuck up
• TTT - Ting tar tid (Norwegian for "things take time")
• W/O - Without
• WB - Welcome back
• WTF - What the fuck

Fanfiction:

• AH - All human
• AU - Alternative universe
• OOC - Out of character
• OS - One shot
• POV - Point of view

Technology:

• IE - Internet Explorer
• FF - Firefox
• OS - Operating system