An assortment of brushes - sizes, some for enamels, some for acrylics, etc. etc.
An assortment of paint. For car modelling, Flat Black, Gloss Black, and Aluminum are critical.
An assortment of putties. 2 part "hard" putties for forming things, other softer putties for finishing.
An assortment of glue. Clear parts glue. Krazy/super/CA glue. General modelling glue. Extra Thin Glue.
Aluminum Tubing for exhausts, etc.
An assortment of sandpaper.
Skewer sticks for holding parts, stirring paint, etc.
Sanding files.
Panel markers. Even if you don't use them to do panel lines, they're great for writing on the plastic.
Many, many more things. I'm trying to think of what I all have in my toolkit and box of bigger things.
EDIT: Add to that
Microsol
Microset
Bare Metal Foil