I bring the same stuff everybody else brings, but more socks and underwear, because clean socks and underwear raise your comfort level significantly, and don't take up much space.
A few other things:
Multitool+ spare (jaki tends to borrow mine!)
Lint roller
first aid kit+ extra box of assorted bandaids
tylenol/ibuprofen/advil (sometimes all three!) for hangovers and food headaches
Ipod
large powerstrip/surge protector, because one outlet per person doesn't work. ever.
a handkerchief
a small but powerful LED flashlight for walkthrough on tuesday.
VISA card. Good luck checking into a hotel without one.
chargers of every variety, same with cables
New for K-Con 2009:
a map with places that sell alcohol and their hours of business in the vicinity.
A better map with GOOD places to eat on it.
box of spare screws, nuts and bolts, safety pins, and fasteners of all shapes and sizes.
a multitude of pens of all shapes and sizes.
A spare projector connection cable
A spare computer/TV power cable
extension cable, 20+ feet, grounded
AA and AAA batteries
USB memory stick, as large as I can find with PDFs of Kcon policies, schedule, con book, maps, installers for VLC and other programs used at con.
DUCT-TAPE! I dont know how I forgot this last year! Same goes for superglue, electrical tape, sheet rock tape, paracord, chemlites
building materials, small pieces of wood, hard plastic and other small things for jury-rigging gaming and other equipment back into working order. (that repaired rockband pedal, the one with the plexiglass sheet superglued to it? that was me)
so basically it boils down to having a backup for everything, and sometimes backups for backups. Spare parts too, whenever available for purchase. "Two is one, one is none." You can chalk this all up to my obsession with preparedness
a vest of some sort that will carry essential items without making me look like a complete dork or Rambo wannabe
thats about it. comments? questions?