I pretty much never travel without both my well-stocked computer case and a carry-on bag. Overseas is more difficult because of restrictive carry-on rules, but that's rare enough I don't worry about that except as the exception and am never happy... Generally wear a travel vest then with lots of the stuff I normally pack in the vest

On person is my Crackberry phone and normal pocket gear including a couple flashlights. Knives going in checked baggage. Almost always have checked baggage for that reason. :-(

Rolling Computer Case includes:

Computer (duh!) w/ charger
Verizon Air Card
Spare battery for Computer
Web-cam
Mouse
Phone Charger
MP3 player and Noise Canceling earbuds
Three paperback books
8.5 x 11 Ruled Notepad
Travel File
Assorted Pens
First Aid Kit with OTC Meds
JIC Prescription Meds
4-inch Scissors
Large Nail Clipper
Duct Tape
Safety Pins
2-3 Flashlights
Digital Camera
Mini-Mate Ionic Air Supply (especially useful when the guy next to you is a heavy smoker and smells like it, or hasn't taken a shower in a week)
2 x Folding N-90 Face Masks
BIC Lighter
Aseptic Packed Tuna (folding spork and paper towels all in ZipLock bag), Lara Bars and other eats
Waterless Towelettes
Back-up Funds and Credit Card (hidden)
colored zipties (to secure checked baggage)
Toiletries in 1 qt. ZipLock bag incl. hand cleaner
Travel Tooth Brush
1 Bottle Water (picked up past security)

In my carry-on:

Minimum of complete dress for next anticipated business day including shoes or boots (which varies depending on what I am supposed to be doing). Sometimes two days, just depends on how bulky it is.
2-days underwear
Personal Grooming Gear
2 packages aseptic packed Tuna and breakfast bars
Medical Kit
Sewing Kit
Duct Tape
Large Clip (holds drapes closed)
Vacuum packed T.P.
3 receptacle 6 ft. Extension Cord
Pocket Survival Pack
2 x Folding N-90 Face Masks

Outerwear, including gloves and cap/hat, I typically carry on person, may stuff an additional layer in carry-on

Sure I am forgetting something....

I can say that I have at various times ended up using virtually everything I carry.