Of the top of my head? Crowbars, claw hammers, sledgehammers, hatchets or axes, box cutters (though not as much with utility knives), flamethrowers, nail guns, probably some variety powered saw I’m not thinking of (though, not chainsaws), flashlights. Though, really, a decent sized wrench is better than nothing.
Crowbars or pry bars are a simple length of solid steel. Usually either round, or flat (the round ones make better weapons because of ergonomics), these are hooked on one end, and both tips flatten to a semi-sharp blade. You can’t whittle with one, but you can certainly use it to take an opponent apart.
Hammers and axes are actually weapons. I used to own an 8lb sledge with a “misuse can result in serous injury or death,” warning label. Because what we really need to do is inform those people who haven’t realized you can use a sledgehammer to kill people, that it’s an option.
Claw hammers are slightly shorter than traditional warhammers, but it’s still the same basic design, just intended for civilian use. As a weapon, it even retains the reverse beak, which will allow the wielder to hook the hammer into their foe.
Hatchets and axes are in roughly the same situation. They’re not identical to real weapons, but they’re close enough that it doesn’t matter.
Box cutters are folding knives with a 3″ to 4″ blade. They’re not an impressive weapon, and some cheap ones might come apart in combat, but it’s a knife, and can get the job done.
Incidentally, longer knives intended for things like clearing brush may be an option (this includes the machete). They’re not intended for use as weapons, but they’re still better than nothing.
Worst case, larger screwdrivers can function as an improvised stiletto. It’s not elegant, it’s not a weapon, but you can probably put that in some poor guy’s neck.
So, if you’ve never spent a lot of time in rural areas, the inclusion of a flamethrower might seem a bit deranged. We’re not talking about military grade combat weapons, these are propane powered torches designed for clearing away dead brush and starting controlled burns. In most states (and I think in Canada) you can purchase small propane powered flamethrowers. You’re not going to want to get into firefights with people using one of these, but, again, if you have no other options.
A similar, slightly more horrifying option, is the thermal lance. This is actually welding equipment, so you’re not going to see this in most hardware stores, but it does exist. The thermal lance is a plasma torch, these are used to cut through reinforced metal. While I’m not sure exactly what this would do to a human being, I’m quite certain the results would be very unpleasant.
Nail guns, at least at short range, might be an option. I’ve honestly never really looked into how viable these are as weapons. You honestly might need tool to skin contact for it to work.
Flare guns are another option. Most commercial flare guns fire a 12gauge shell. The flares themselves are very low power, but will burn on contact. The gun itself can’t use normal 12gauge shells, it will explode. Flare shells can be loaded into most (or all) 12gauge shotguns, however, semi-automatic shotguns will not cycle between shots because there’s insufficient force. Still, getting hit with one of these will probably kill you.
The problem with chainsaws is that the viscera will get pulled into the motor and jam the system. This is probably true of most powered saws, though I’ve never really looked into it. That said, if your character just needs to kill a single foe, a powered saw should get the job done.
It might have sounded like a joke, but a loaded flashlight can be a pretty formidable choice. Particularly something like a D-Cell Maglite. These are practically a weighted baton in the right hands, and can do a lot of damage on impact.
The thing that’s probably messing with you a bit is, nothing on the above list (with the possible exception of the hammers and axes) makes a particularly good weapon. (Also, if you’re actually planning to write a scene in a hardware store, some of the above items won’t be in a functional state on the floor. Sadly flamethrowers, thermal lances, and a few others come, “some assembly required.”)
With enough creativity you can probably kill someone with most of the objects in your environment. Improvised weapons are about finding what you have immediate access to, and deciding which is the best option from that list. It’s not about getting “a good weapon,” just finding something you can kludge into one.
A character who takes a flat head screwdriver around as their weapon of choice is going to come across as slightly goofy. A character who, while struggling to fight off an attacker, grabs a nearby screwdriver and drives it through their foe’s neck, won’t. That’s the difference.
A lot of the time, we’ll get a question like, “I want my character’s weapon of choice to be a machete,” at which point I go into the entire discussion of how, “it’s a tool,” and “it’s not designed for combat.” It’s not that you can’t kill someone with one, but it shouldn’t be your characters first choice when dealing with a situation. It’s not a good weapon, but sometimes it’s the only option your character has.