People are always looking for the next big way to get something from nothing. We've seen it again and again in the financial world, with banks filling their coffers from trading toxic assets that weren't actually worth anything to begin with, and it's there in the history of space exploration too. Space is big, space is vast, space takes a long time to get through; just think of how much easier everything could be if you could get through it fast. It's too bad space doesn't work the same way as Earth does - if you want to move through a vacuum, the only way we've got is to throw crap out the back of your ship in a stunning proof of the Newtonian principles of action and reaction.
People tend to chafe at this. As a result, there's never any shortage of inventors who claim to have built something that would shatter the foundations of modern physics - a reactionless drive, a mode of thrust that doesn't throw any crap out the back of the ship. Considering that ships would need to carry a hell of a lot of crap with them to get anywhere in a reasonable time, a reactionless drive would really make things a hell of a lot easier for those spaceship engineers.
Back in the 1950s and 1960s, the standard bearer of the field was the Dean drive, a purported reactionless thruster that was championed by John W. Campbell and G. Harry Stine over at Analog, presumably because they really wanted to believe. The drive has since been demonstrated to not be a drive at all, its thrust illusory based on friction with what it's sitting on... not exactly workable for a space drive.
But fear not! We may yet be able to transform submarines into spaceships! Because, ladeez and germs, here comes... the QDrive! It is, according to the website, some sort of radiation pressure thruster designed in such a way that a "differential in radiation pressure generates an unbalanced force that creates thrust... without use of propellant." It's being developed by Cannae LLC, a company apparently focused solely around the development of the drive, presumably deriving its name from the Battle of Cannae out of the hope that the modern scientific orthodoxy will play the part of the Romans, and that the metaphor will not go any further than that... after all, Rome eventually annihilated Carthage.
That's incredible, folks! Just imagine what you could do with something like this! Those QDrive people must be just bursting with ideas...
The most important sentence in the entire sphere of science, I believe, is this: extraordinary claims require extraordinary evidence. If you claim that there's an invisible pink dragon living in your garage, you'd better have some good information backing it up if people don't want to think you're crazy. If you claim that pumping billions of tons of carbon dioxide into the atmosphere every year is having no effect on the planetary climate whatsoever, it behooves you to offer an explanation for what it's doing instead. And if you've put together a machine that appears to give you something for nothing, it's your responsibility to look very, very closely at it.
I'll admit, not all thrust modes use propellant in the "throw crap out the back of the ship" sense. Solar sails are the usual suspect here; they generate thrust by the pressure of radiation in the solar wind. I note that this is rather similar to the explanation as to how the QDrive works - all I'll say is that not only am I not a physicist, I don't even play one on TV.
Nevertheless, for something like this it's only proper to be skeptical. If an actual reactionless drive ever comes along, it would thrust past skepticism.