A (usually) cordless handheld vacuum cleaner manufactured by Black & Decker. Some people use the name to refer to any handheld vacuum cleaner, much like using "Kleenex" instead of tissue or "Xerox machine" instead of copy machine. This is called trademark dilution, as anonymous softlinkers have kindly pointed out. It is also called a copyright misnomer, as more anonymous softlinkers (possibly the same ones) have kindly pointed out. What I'd do without these people, I don't know. Anyway, Dustbusters look somewhat blocky, knockoffs usually look curved and slightly phallic.
Also a common nickname for the nVidia GeForce FX 5800 Ultra, a poorly designed and unpopular video card. The GeForce FX 5800 Ultra had a big cooler on it consisting of an oversized copper heatsink and a blower that was not only enormous (you couldn't install anything in the PCI slot below the card) but also obnoxiously loud and whiny. Adding insult to injury was that the card was about as fast as the ATi Radeon 9700 Pro that had come before it (slower in newer games, actually, and surely that's what people buy these things for?) and was much more expensive. And since insult and injury apparently weren't enough, nVidia added image quality-reducing optimizations to their drivers to make their card look better in comparison to the Radeon 9700 Pro. They later removed the "cheats" from their drivers, but have had trouble living them (and the dustbuster itself) down. And yes, the thing really does sound almost exactly like an early-model Black & Decker Dustbuster, though since it is sealed in a plastic case (which is itself sealed in a big metal case) it is not quite as loud.
See also Itanic, or if you're more interested in the vacuum cleaner that doesn't have a free piece of shit attached to it see also Dirt Devil.