Electro-Magnetic Guns
While playing with my can crusher, I noticed that a can placed off center tended to be pushed out of the solenoid.  A little searching of the patent literature convinced me that I had inadvertently created a very poor, single stage, coil gun.  Presented below is a summary of what I have found so far.

Propellant powered guns are typically limited to muzzle velocities on the order of 2,000 meters per second.  This limit is inherent to the use of expanding gas to drive the projectile down a barrel.  Barrels simply can't withstand the temperatures and pressures required for higher expansion rates of the propellant combustion products (normally CO2 and NOx).  One attempt at a gun for higher velocities used differential pistons (a large one, driven by methane/oxygen combustion, connected to a small one for compression of the drive gas) to provide a high pressure of hydrogen gas (hydrogen is the lightest, and hence fastest expanding, of all gasses).  While some success was achieved, the apparatus was cumbersome and the velocities were still limited.  For some applications, particularly orbital launching, this is insufficient (earth escape velocity is 11,200 m/s).

Two basic types of electromagnetic gun are described in the patent literature, the rail gun and the coil gun.  Both use stored energy sources to produce a large magnetic field and a high electric current through a driving armature.  The interaction of the current with the magnetic field generates a force which propels the armature (and any projectile connected to it).  Beyond that, they differ substantially, and each has practical difficulties which has prevented them from being more than laboratory curiosities.

Schematics for Rail and Coil Guns