These are not unique to fanfic, but as newbies keep asking about them, here they are... This is NOT an exhaustive list, just slang commonly adopted by fandom.

• <.< : *eyedart* (guilty or up to no good)
• >.> : variation of the above
• <3 : Heart (tilt sideways to view)
• ASL?, A/S/L?: Age/Sex/Location? *
• AFAIC: As Far As I'm Concerned
• AFAIK: As Far As I Know
• AFAIR: As Far As I Remember
• AFK: Away From Keyboard
• AIM: AOL Instant Messenger
• ATM : At The Moment
• BION: Believe It Or Not
• BBL: Be Back Later
• BRB: Be Right Back
• BTW: By The Way
• f2f, F2F: Face To Face
• FAQ: Frequently Asked Question(s)
• FOAD: Fuck Off And Die
• FOAF: Friend Of A Friend
• FWIW: For What It's Worth
• FYI: For Your Information
• G2G: Got To Go
• GD&R: Grinning, Ducking, & Running
• GMTA: Great Minds Think Alike
• HTH: Hope This Helps
• IIRC: If I Recall/Remember Correctly
• IME: In My Experience
• IMHO: In My Humble Opinion
• IMNSHO: In My Not So Humble Opinion
• IMO: In My Opinion
• IRC: Internet Relay Chat
• IRL: In Real Life (as opposed to the Internet)
• KISS: Keep It Simple, Stupid
• LLAP, LL&P: Live Long And Prosper
• LMK: Let Me Know
• LOL: Laughing Out Loud
• MMMISAT: Mmm, I Smell A Troll (an alert that someone is trolling)
• NG: Newsgroup
• N/P, NP: No Problem
• OIC: Oh, I See
• OMG: Oh My God
• OMFG: Oh My Fucking God (stronger form)
• OTOH: On The Other Hand
• PDA: Public Display of Affection
• PITA: Pain In The Ass
• POS: Parent(s) Over Shoulder (aka "Shhh, don't get me in trouble!")
• ROFL: Rolling On Floor Laughing (incorrect version of ROTFL)
• RL: Real Life (as opposed to the Internet)
• ROTFL: Rolling On The Floor Laughing
• ROTFLMAO: Rolling On The Floor Laughing My Ass Off
• RTF: Read The FAQ! (common response to a stupid question)
• RTFM: Read The Fucking Manual/Message (aka "DUH!")
• SO: Significant Other
• TANSTAAFL: There Ain't No Such Thing As A Free Lunch ("Oh well, tough luck")
• tl;dr : too long; didn't read
• TTFN: Ta-Ta For Now
• TTYL: Talk To You Later
• W/: With
• WB: Welcome Back
• WTH: What The Hell
• WTF: What The Fuck
• WYSIWYG: What You See Is What You Get
• YHBT: You Have Been Trolled (said if you took a troll seriously)
• YMMV: Your Mileage May Vary ("it's okay if you feel differently about this")

• <G> or *G*: Grin
• <EG> or *EG*: Evil Grin
• <BG> or *BG*: Big Grin
• <VBG> or *VBG*: Very Big Grin
• <BEG> or *BEG*: Big Evil Grin
• <BFEG> or *BFEG*: Figure it out ;)

* This acronym, a crude request to scope out other chatters as potential cybersex partners, is only used by mundanes (usually AOLers, who don't know any better) who mistake a fan chatroom for a pick-up spot, and usually results in expulsion. Fans do not tolerate obnoxious outsiders.