The knife and flash light are on me wherever I go. in the trunk I have a small set of the basic combo wrenches, basic sockets in 1/4 3/8 and 1/2, raised panel ratchet in 3/8 with adaptors for 1/4 and 1/2, pliers, adjustable wrench, and a breaker bar in 1/2. I made a "custom" holder for everything. It's fiberglass modules that sit in the spare tire and i shadowed it all with foam inserts into the fiberglass. Doesn't take up any cargo space and keeps it all organized. I keep a first aid kit in the trunk too, but no room in the holder for that one. Another thing I keep is a bottle jack, because I don't like the scissor jack that the car has.
I do drive a ford, so this is all needed as it likes to break down at the most inopportune moments.
I'm sure most of you on this board are also "go to" guys when a friends car breaks down on the side of the road, so it's convenient to have all this stuff already packed when you need to go rescue someone.