This article is from the Radio Control (R/C) Flying FAQ, by Shamim Mohamed email@example.com with numerous contributions by others.
There are a million accessories that you can buy. There are a relative few
that are indispensible, or almost so. I'd put the following items on the
short list: a prop balancer, a pitch gauge, a pair of ball link pliers, and
a receiver battery tester. You will need a standard assortment of tools
such as needle nose pliers, screw drivers, hex wrenches, etc. You'll also
need a starter and starter battery.