Easy2Boot - http://www.easy2boot.com: Use this to build your USB bootable. You prep the drive with it, and then just drag and drop ISOs :D So easy :D thx, /u/craig131!
TRON - http://bmrf.org/repos/tron/ (official repo) : TRON is a great package allowing you to check systems, repair many types of damage to windows, and remove virii and malware. it is in this subreddit, if you care to look.
Portable Apps - http://www.portableapps.com : A great suite of portable apps you can run off your USB key. Firefox, Chrome, OpenOffice, ClamAV, and many others. Even games, but this is a tech question...
Sysinternals Suite - http://technet.microsoft.com/en-us/sysinternals/bb545021.aspx - The name is well known, and is quite an impressive suit. It speaks for itself.
NTPasswd - Available through Sardu. : I've run into a LOT of customers who REFUSE to give me their user OR admin passwords, and it's wound up being required to fix and backup their junk. Use ONLY for legit reasons, please.
Ninite - Http://www.ninite.com : A wonderful package site that lets you choose productivity, AV, antimalware, antispyware, and other apps and runtimes in ONE stub downloader. Simply choose the apps you want, and click "downlaod installer" It will download a file less than 300k in size that you can run on an internet connected windows PC, and it will download and install the applications you chose. You can even have task scheduler run it in silent, if you ahve the PRO subscription, and it will update the apps, as well.
33 piece screwdriver set, made by "Pittsburgh" - http://www.harborfreight.com/hand-tools/screwdrivers/comfort-grip-screwdriver-set-33-pc-61255.html : Nice set, magnetized tips, and a socket driver with bits is included. Useful for most tasks.
34 piece PRECISION screwdriver set - http://www.harborfreight.com/hand-tools/screwdrivers/33-piece-precision-screwdriver-set-93916.html : Once again, very usefull for smaller and small ODD bit types of screws. Made by "Pittsburgh" and even includes a flex shaft for getting around corners, if need be.
100 piece SECURITY bit set. - http://www.harborfreight.com/100-piece-security-bit-set-68457.html : Trust me, I hate those little screws with the BUMP in the middle. This takes care of most, and even Apple devices in many cases. Not too shabby for the price.
14 function digital multimeter - http://www.harborfreight.com/5-in-1-digital-multimeter-98674.html : Nice meter. Not a FLUKE, but not 50.2 googol dollars, either.
5 Function digital clamp on meter - http://www.harborfreight.com/digital-clamp-on-multimeter-95652.html : Used for measuring current draw, is all I use it for.
3 set of SAE/METRIX/STAR hex keys. - http://www.harborfreight.com/pack-of-3-folding-star-bit-and-sae-metric-hex-keys-94905.html : For the odd time you need hex keys.
2TB HDD (USB 3.0, please) - Available at Newegg, of course. : Used for backing up customer data in the field as necessary.
Headset, plug in type. - On web, at radio shack, etc : Used for testing audio on systems. And keeping the peace.
Fold up tool bag - http://www.harborfreight.com/20-in-rollaway-tool-bag-61925.html : Used to hold your kit.
LAPTOP - Anywhere fine computers are sold : I can't stress this enough. I've been in situations where a tech has FORGOTTEN to bring a laptop, and had to borrow mine. It is SO great to be able to test the internet connection when they say it is down, and to SSH/FTP/Download needed things. GET ONE :D
MOUSE : Nothing sucks more for doing work quickly than a laptop trackpad, IMHO. Also, great for testing USB ports. GET A CHEAP ONE. Don't kill your Logitech G600!
A 2.1 Amp wall charger with at LEAST one USB port. - Available on Web : This can be a life saver!
Various USB charger cables. - Http://www.monoprice.com - Another life saving thing.
A VGA, a DVI, a HDMI, and a DISPLAYPORT cable set - Http://www.monoprice.com : For those days when it MIGHT be the cable.
A small spare LCD panel monitor! - Http://www.newegg.com : For testing those no display systems and laptops.
A spare USB keyboard - Http://www.newegg.com : Pick your fave junker :D
A USB battery pack! - Http://www.monoprice.com or Http://www.amazon.com : This thing has saved me a few times on the road when I forgot...
Cigarette lighter USB charger - Http://www.amazon.com : This thing is wonderful :) Make sure to get a 2 amp one :D
DC to AC car cigarette lighter inverter - http://www.newegg.com/Product/Product.aspx?Item=N82E16882021190 : Yes. Laptops DO run out charge on the road.
A 4G Hotspot - Various wireless cell carriers : There are plenty of job sites without even so much as DIAL UP internet, and you desperately need a driver.
A cellular telephone - Various cellular wireless carriers : Phone calls, boss calls, 911, etc.
Universal laptop charger kit - http://www.rocketfishproducts.com/products/computer-accessories/RF-ACU9025.html - $69.99! I know it seems pricey, but it is honestly GREAT when people have a laptop that CAN'T charge due to missing brick, or WON'T charge and it MIGHT be the brick. Plus, you can leave your MAIN laptop charger at home!
8GB USB thumb drives - http://www.microcenter.com/product/280099/8GB_USB_20_Flash_Drive: As pointed out, later, they can be obtained from Microcenter Online for $4.99 USD.
A lunch! Be sure to grab your favorite HEALTHIER snacks, like granola bars, REAL fruit juice, water, etc. If it will be a significant stay, be sure to grab sandwiches or lunch meat, bread, and condiments. Also, grab a cooler and 4 ice packs to hold it all and keep it cool. Bacteria BAD!
Pens, pencils, sharpie markers, and WRITABLE TAPE. Also throw in velcro ties for tidying up cables. It's a labor of love in some paces, but clients do appreciate it! Do NOT, however, velcor AC or DC cables together with network cables ;)
Pliers and wire cutters! You never know when something will get stubborn!
GMER on you main thumb drive! - http://www.gmer.net ! :Rootkit detection and removal kit.
HookShark - http://www.woodmann.com/collaborative/tools/index.php/HookShark - HookShark is a detector of installed hooks and patches installed on the system (only usermode for now). It scans through the code-section of every loaded module of each running process and compares it with the file-image. If it detects discrepancies it tries to determine the type of hook or patch and reports it to the user.
http://drp.su/ - DriverPack Solutions. : Download the DVD ISO via bittorrent, and there is a TON of drivers on the image. You do NOT need to use the installer. Just point to the appropriate driver directory and search subfolders. You should be golden.
Technibble support contract package and checklists. - http://www.techinbble.com : A good package of paperwork, covering your but in most cases. Grab it, and edit it for your business name, etc :D It is no longer provided, but there are similar packages :D
Network cable tester from http://www.amazon.com : It cost me $5USD and is a definite help
Platinum Tools EZ-RJ45 crimper, Platinum Tools EZ-RJ45 ends, boots, and a box of category 5E. : Http://www.amazon.com - This tool, ends, and cable are GREAT for running new cat 5e effectively. The ends are open on both ends, and you can slide the wires through in the proper order, crimp, and trim, all in one shot. Worth the price.
RG6 Quad shielded cable spool, RG6Quad compression fittings, stripper, and compression tool : http://www.homedepot.com - I use compression fittings for cable modem cable runs. I love the weatherproof ones, and they are easy to set up.
A "spudger" - https://www.ifixit.com/Store/Tools/iSesamo-Opening-Tool/IF145-130 : This tool is GREAT for popping laptops. Thanks /u/wanderingbilby for reminding me!
A flashlight. Preferably, a head mount one will be included, as well as small handheld, and normal sized. I prefer LED ones :D
T shaped Wall Tap (US), 2 to 3 prong adapter (US), and Outlet tester (US) - http://www.amazon.com/gp/product/B0081A4GEC?pldnSite=1 , http://www.amazon.com/gp/product/B0027B3KO8?pldnSite=1 , http://www.amazon.com/gp/product/B000X3FSAQ?pldnSite=1 : Heaven forbid, you plug something in of your kit and it POPS :(