few things- my "must have" is a 12 volt test light- to load circuits in a way a DMM will not. An assortment of 18" leads with alligator clips on either end.
A good DMM along with several cheap models- I use a Fluke 116 with a Tenma AC/DC current clamp, a cheap DMM that Dinosaur Electronics handed out at a trade show, and a test kit I bought on sale from Radio Shack with a decent DMM (does capacitance and frequency), non contact voltage detector, light and sound continuity tester and thermocouple temperature tester- all around $50. Also have an old Archer Analog meter- don't use it often, but when I need it, nothing else does as well.
Oh yeah- a big box of replacement fuses, as well :)
