PA Toolkit

It's not vital but there's a few things you should have around that will make running a sound desk that little bit easier:

Headphones
Having your own pair is vital, it doesn't matter what they look like they just need to feel comfortable and sound good. DJ headphones can be particularly good as you can turn one away from your ear so you can hear both what's in the room and what's going through the system.
Gaffa/ Duct Tape!
You can never have enough of it and it will disappear so try to hide it. It's great because you'll need it to make cables safe, suspend them over doors and fix just about anything
Masking Tape
Obviously less adhesive than Duct Tape it is great for labelling things. For example putting it on the mixing desk, labelling a cable at both ends, marking things like broken equipment that needs fixing, etc.
Multitool
A screwdriver, pliers and a knife can sort out a lot of things and a multitool such as a Gerber or Leatherman has them all. Examples are tightening up loose screws, fixing tough mic stands, cutting gaffa, etc.
Batteries
Its the simplest things that makes things work well. An extra couple of 9V batteries for wireless mics are always useful even if you think there are enough around. The last thing you want to do is run out for batteries when doing a setup.