I put my spare tire above the cargo space floor so I have a lot of room down there.
Whoo boy, this could be a long list...
Recovery-wise, I have the stock jack, detachable tow point, clevis fasteners, two tow straps (of different lenghts), my trustworthy old entrenchment tool, a cheap come-along I feel rather sceptical about, and a saw. Above the cargo space door there is a lenght of 2x4 I use as a jack base and a longer, hiking-type shovel. (Shovel is tied to the rear seat head rests with paracord so it is quite well out of the way.)
Then there's two poncho liners (the rain ponchos are attached to the first aid bag behind front passengers seat) and packable water resistant jacket and trousers for warmth, emergency use etc. There are also at least two pairs of surplus Bundeswehr combat gloves - these make for pretty good work gloves (they are none too weather-resistant on their own however).
Two six-packs of 0.5 liter bottles for a total of 6 liters of water. For emergencies, yes, but also for plain old traveling, especially with a dog - cleaning up messes, drinking, making food... (A .50 caliber ammo box containing hiking food is located above the floor, as the shape of the box makes it a tad difficult to make it fit under it.) I figure that if we use a couple of bottles for non-emergency situations, that still leaves us with plenty for unforeseen circumstances.
A tire iron is an obvious thing to have in there.
And then there's the duffel bag with all sorts of stuff in it - jumper cables, Esbit stove, mess kit, a durable fixed blade knife, firemaking tools, tinder, batteries, high-visibility vests (two), duct tape, cargo strap... (There is also another cargo strap is already in place for strapping a cooler to the rear seat.)
Not included in that list are the fire extinguisher kept high right against the rear seat (so it can be reached by opening either the rear door or from the rear hatch); afore-mentioned first aid kit (I like to keep this and the fire extinguisher where I can get to them quickly - as someone mentioned above, the problem with under-the-floor gear is getting to it when you are carrying a bunch of stuff) which is built into a musset bag type thing which is hanging behind the front passengers door; my SOG tomahawk (this I could keep in the cargo space, but it fits so neatly in the gap between the drivers seat and body of the vehicle that it would be a shame not to use that space); maps and a list of commonly used CB radio channels according to area, a few critical fuses in the glove box (probably going to relocate them to the duffel bag once I get around to it). A compass rides with me in my backpack. PMR radio phones are in the glove box - should probably get a spare for the Jeep one of these days as well. Cell phone charger already plugged into the 12V socket. Ice scrapers and snow brushes here and there. A huge light stick that would function nicely as a signaling device in a dark road goes between the front passenger seat and vehicle body - kept there so I don't have to dig it up from under the cargo space floor before I can use it to signal passing travelers on a dark road.
I am probably omitting something from this list, too.
