If you are a computer technician and you do service calls onsite there are certain things you should always carry with you in your vehicle. Getting to a job and not having a spare power supply or a spare SATA cable can be real pain. Not to mention lost profits on the upsell, and having to drive to a store and over pay for it. The combination of lost time and money will make having all of this stuff well worth it.
Some of these items I specify should not be top of the line. If your car gets broken into or the sun turns your car into a 400 degree oven you may lose some of the items. Store all of this in the darkest part of your vehicle so the sun won’t damage them. The hard drives and anything else fragile should be packaged properly to take the abuse of riding in a car. I will tell you that 90% of service calls can be taken care of with a screwdriver and a USB jump drive packed with repair utilities. I have put together a list of items and this is what I’ve come up with.
Molex to SATA adapter splitter (below)

Molex to SATA
2.5″ hard drive SATA (1)
3.5″ hard drive SATA (1)
USB to SATA hard drive caddy (below)

USB to SATA dock
80mm fans (assortment of fans)
Network cards (PCI and PCI Express)
Network cable crimp kit
Thermal Grease (below)

Thermal Grease
PCI Wireless card
USB Wireless stick
Anti-static bags (below)

Anti-static bag
Large selection of plastic cable ties
Small air compressor or cans of air
5mm nut driver – grind down so wall is thin all around. This is used to tighten stand offs and VGA nuts.
Electronics screwdriver (Wiha 269 PH1 + 4.0 slotted is best)

Wiha screwdriver
USB A to B cable at least 6ft long
HDMI cables at least 8 ft long
Power strip / surge strip (below)

power strip
Network cables many different lengths
Phone cable
Powered USB hub (below)

USB hub
5 port switch
Wireless router
Large paper clips (to open DVD drawer)
USB External hard drive (below)

USB external hard drive
Cheap digital camera
Pen and Notepad
Business cards, lots of cards (below)

Business Card
Small needle nose pliers (Xcelite 378M below)

Xcelite 378m
Small shearcutter (Xcelite 170M below)

Xcelite 170m
Roll of electrical tape
Flashlight with spare batteries
Power supply tester
ATX power supply (below)

ATX Power Supply
SATA and IDE cables
Large selection coarse thread screws for 3.5″ hard drives
Large selection fine thread screws for DVD drives (below)

Assortment of screws
Low end PCI-e video card
Operating system discs
USB media card reader
Hot glue gun
Spare DVD Drive
Small solid copper wire
Cheap laptop or netbook
Paper towels and Glass plus cleaner (I’ve found Glass Plus to be the best)

Glass Plus
7 in 1 screwdriver (below)

7-in-1-screwdriver
Lil Chizler plastic scraper spudge tool (below)

Lil Chizzler
USB Jump drive updated with Ketarin top 50 (below)

Ketarin