I travel US domestic mainly. I keep my iPhone (boarding pass) and passport (ID) in my pocket in line until checked by TSA, then put them in my backpack for screening. I like my pockets empty on the plane.

I'll have a roll-aboard with me with up to 3 days' clothes and toiletries, or a messenger bag with 1 day's clothes and toiletries if I check a bag.

This is in my backpack-style underseat carry-on bag, which I call my "office in a bag":

laptop (in pocket for screening without removal) and power supply
notebook & travel document folder
mini USB cable
USB serial cable
5' CAT5 cable
Logitech presenter in case
110V - USB x2 adapter
about 6 - 8 pens from various hotels & a Sharpie
two red poker chips
my business card case & one in the "ID" pocket
Garmin nuvi 350 in case, car power cable, windshield holder (PPA parking ticket in case pocket)
iPhone 5 & power/USB cable (work)
Samsung Galaxy Nexus & power/USB cable (personal)
spare Galaxy Nexus battery (in baggie, of course)
car 12V DC - micro-USB cable
Passport
4 Band-Aids
comb
a half roll of Tums
a 10 pack of 200 mg Advil
a pack of Wrigley's spearmint gum
a small pad of post-it notes
2 USB drives (4GB & 1 GB)
small (6") screwdriver
6' keychain tape measure
my wallet
my sunglasses
my house and car keys
noise cancelling earbud headphones
normal earbud headphones
airplane headphone adapter
~$4 in change
my travel pillow (comes out right before boarding)
I may have other tools/gadgets with me, depending on the purpose of my trip (tech support engineer).