Telescoping magnet

The most useful tool that I carry and use every day is a MagLite Mini 2 AAA cell flashlight.

zip ties. They're fairly good for keeping things neat, but PLEASE use them sparingly.

Nothing worse than coming in behind someone that has the "police issue handcuff model" to tie cables together every two inches, and they're cranked down so tight it's almost cutting into the cords.

Cause, you know, once a PC is setup there's never gonna be a reason where you're going to need move it, or swap it out.

Nice tips on the rubber bands, and some of the software utilities by the way.

I have a cybertool with a torch built in, use it a lot, in fact it is in a pouch on my belt.
If you have the space, a electric screwdriver is handy. Some places can be difficult to reach with a normal screwdriver.
You can use your smartphone for troubleshooting fiber as well.

Tums and Advil - no seriously. A linux cd in case you need to boot to another OS