What are some of the essential tools that you keep with you for each project?

Mine list is pretty simple:

Needle Gauge, which is also a ruler
Tape Measure
Stitch Markers (clips and rings)
Crochet Hook (for fixing mistakes)
Row Counter (I use the County app on my phone)
Scrap Yarn
Scissors
Pen
Small Notepad
Cable Needles
Nail File
Hand Lotion
Chapstick

I keep hand lotion and Chapstick everywhere, though.