US patent number 4050426 is a patent that seems to fail the "usability" criteria for patentability in the USA. Note: this is not a software patent.
A method and apparatus for treating liquid fuel in an internal combustion engine. The fuel treating device is connected in the fuel line leading to the carburetor or other fuel/air mixing apparatus and comprises an elongated hollow outer casing made of a magnetic material, an elongated inner casing made of non-magnetic material positioned within the outer casing and a single elongated magnet received in the inner casing, the maget
[sic] having at least three longitudinally spaced apart sections of alternating, north and south polarity. Means are provided for supporting the inner casing in spaced relation to the outer casing to form an annular chamber therebetween
[sic], in which chamber the lines of flux are preferably radial. The device further includes an inlet and an outlet located near opposite ends of the annular chamber for providing fluid flow from the fuel line through the chamber.
The US Patent Office requires that the device be "useful". This device cannot work as described, as magnets of the type described have no effect on hydrocarbons. Tests have been carried out by several organizations:
- CNN - No shortcuts to saving money on gas
- Mythbusters - Episode 53 Part 2 - The Great Gas Conspiracy
- Popular Mechanics - Looking For A Miracle: We Test Automotive 'Fuel Savers'
 Final Note
While magnets can affect hydrogen in theory (see Bussard Ramjet), the size of magnet needed to produce a field that strong would make it impractical to build on a planetary surface. This does not however stop dreamers, most of whom know no science, from trying magnets. There's something about magnets that just draws peoples attention, and this is illustrated by there being several thousand "Fuel Magnet" patents in the USPTO database.