By far, the most useful household tool (at least in my house) is the POWER DRILL. Yes, the hammer comes in handy a lot, the stud finder, the level, needlenose pliers, jar of miscellaneous screws, etc - all are useful and every household should have them - but the power drill (for drilling, unscrewing, screwing (screws), and torturing naive tourists) seems to be brought out every other day around here.
There might be a few out there that have bought into the hype that duct tape is one of those things to always keep within reach - that duct tape alone can keep a house functioning well into the post-apocalypse. Frankly, I almost never use duct tape for anything other than making last-minute costumes for Halloween (or binding naive tourists). Mind you, power drills won't be too useful when the infrastructure of society collapses - but still, POWER DRILLS, baby -POWER DRILLS!