I recently sprang for a handheld vacuum and it was probably the best 50-or-so-bucks I've ever spent-for me at least, it has been pretty revolutionary.
It's just an incredibly handy thing to have when you don't feel like lugging out your heavier, clunky canister or upright vacuum (which I never do). Most are cordless these days too, eradicating the need to untangle and fuss with any wires or plugs. It's brilliant for sucking up dirt on and underneath floor mats inside a car, for running over the sofa seat cushions and back pillows (especially if you have a pet), tidying up before guests come over, for picking up basically any kind of little spill or mess, for getting into tight corners of a room… I've had mine for less than a month and have pulled it out at least three or four times a week for one reason or another.
What's the cleaning tool your most fanatical about?