Hmm, for me it depends on what game I’m running, where I’m running it, and are we including prep resources…
Since all of my current gaming is online, Play by Post or Roll20/Hangouts, here is the most important resources I have handy:
Computer with the following things handy
Rules (OD&D or Classic Traveller)
Monster & Treasure Assortment (D&D)
Supplement 4 (CT)
My version of the Classic Traveller Character Generator web tool (CT)
Setting (Wilderlands of High Fantasy for OD&D, Wine Dark Rift home brew setting for CT)
Adventure 10 – Safari Ship (two of my CT campaigns use this ship…)
Tons of other stuff handy
Dice, paper, pencils (I am not 100% digital…)
Printed maps (dungeons, setting) and keys
Various game notes
Gary Gygax’s Extraordinary Book of Names
For Traveller, a binder of useful articles (index here: https://docs.google.com/document/d/1hsD8WYkLZpEWnG1lVpodV6H2bbAObyMNIck0eYJblno/edit?usp=sharing), for OD&D I also have some various printouts, I don’t reference them nearly as much.
For OD&D, Judges Guild’s Ready Ref Sheets, also other editions of the game (Holmes Basic, AD&D – for Traveller I also have Mongoose 1st ed. handy)