Ok my gadget for today is a cordless vacuum cleaner
with beater brush.
I've posted before about the
Swiffer Sweeper Vac, which is great for pet hair and dust on hardwood.
This gadget is more for carpet and rugs -- especially if you have pet hair.
There are a few of these models out there, and I've owned two.
The one I have now is a
Hoover Linx cordless, which retails for about $150 but which I got for $50 on ebay because it was half-broken.

Previously I had a black+decker one that was almost as good.
The key feature of this device is that it is cordless, which for me means I'm actually likely to use it -- as I hate dealing with cords. And that it has a motorized beater brush, which is essential for rugs/carpets.
You should know that in terms of suction, a cordless vacuum does not even come close to the power of a corded model -- in fact I would go so far as to say that the cordless models are virtually useless as a replacement vacuum for hard flooring work (i'd prefer the swiffer sweeper), BUT if you have carpet/rug and want something to regularly pick up pet hair, the motorized beater brushes work well and these are very useful devices. So again, this is a narrow use gadget in my opinion, but one that does it's particular job well and with extreme convenience.