These are mostly used abbreviations on online forums.

- A -
AFAIK - As far as I know
AFK - Away from keyboard
AR - After rebate

- B -
B&M - Brick and mortar store (A physical retail store as opposed to an online merchant)
B1G1 or BOGO - Buy one get one free
BB - Best Buy
BBL - Be back later
BBIAB - Be back in a bit
BRB - Be right back
BTW - By the way
BVG - Best Value Guarantee (Offered by some credit card companies)

- C -
CC - Circuit City
CC - Credit card
CPN - Coupon
CSR - Customer service representative
CYA - See ya
CYM - Check your mail

- D -
DD - Deal discussion
DOA - Dead on Arrival

- F -
FAR - Free After Rebate
FWIW - For what it's worth

- G -
GC - Gift certificate
GDA - Good deal at

- H -
HD - Hot deal
HSN - Home Shopping Network

- I -
IRC - Internet Relay Chat
IIRC - If I Recall Correctly
IMAO - In My Arrogant Opinion
IMHO - In My Honest/Humble Opinion
IMO - In my opinion
IR - Instant rebate
IRL - In Real Life

- L -
LAN - Local Area Network
LOL - Laughing out loud
LMAO - Laughing my ass off
LMK - Let me know

- M -
MFG - Manufacturer
MFR - Manufacturer
MIR - Mail in rebate

- N -
NIC - Network Interface Card

- O -
OD - Office Depot
OEM - Original Equipment Manufacturer
OM - Office Max
OOS - Out of stock
OP - Original Poster (author of thread)
OT - Off topic

- P -
PM - Price match
PM - Private Message

- R -
RAOK - Random act of kindness
RMA - Return Merchandise Authorization.
ROFL - Rolling on floor laughing

- S -
SASE - Self addressed stamped envelope
S/H - Shipping and handling

- T -
TIA - Thanks in advance
TRU - Toys R Us
TTYL - talk to you later

- W -
WAN - Wide Area Network

- Y -
YAFTS - Yet another free T-Shirt
YGM - You've got mail
YGPM - You've got a private message
YMMV - Your mileage may vary