Other than the hand tools listed above, my absolute must-have tool is my magnetic parts bowl. They're often a freebie giveaway at Harbor Freight with a super coupon.
I have 2 or 3 stuck to the siderail of a game I'm shopping out. Acorn nuts and star posts in one, long metal posts in another, and so on. Keeps the parts within reach, and keeps the metal parts from falling out.