Haven't updated in a few days, had a lot of tests, but this should be an interesting post!
There's an engine prototype already built called the VASIMR or Variable Specific Impulse Magnetoplasma Rocket. It is an exciting advance in rocketry because it is capable of getting us to Mars in under a month, or Jupiter in under a year. For reference, currently used rockets would take about 9 months to go to Mars. The company has also signed a contract with NASA to test the rocket on the ISS by 2013!
This is how it works. Radio waves are aimed at hydrogen gas contained inside. It resonates with the gas giving it a net charge and heating it. It's somewhat similar to how a microwave is tuned to resonate with water, heating it up. It heats the gas to 1,000,000 F and then it is shot out the back (propelling the vehicle forward) using a strong magnetic field. You may hear things like this and wonder how exactly that works. Well, the way electromagnetism works is that a magnetic field goes in a certain direction, say, wraps around an object. Charged particles (or a current carrying wire, like in a wind turbine) go perpendicular to the magnetic field. So, if you aim the magnetic field in the proper direction, you can aim the charge, or plasma, in the desired direction, as well.
A magnetic field must be used, of course, because the plasma needs to be concentrated and shot in a precise direction to get the craft to move as desired. Materials don't function very well at these temperatures. The net result of 3 of these engines on one craft is Mars in under a month, and it's available now. One drawback, and irony, is that the material the magnets are made of is very important. Standard materials won't work because they are too heavy to get into space economically. Light superconducting materials are necessary. The irony is that they don't work at room temperature, in fact they need to be at ~ -300 F, about the temperature of liquid nitrogen. So super hot plasma is being contained by super cool magnets! Another potential problem is the magnetic field's affect on equipment. They were able to orient two magnetic fields opposite each other to cancel out the effects of this. It works similarly to magnets repelling one anther. The field lines avoid each other and diverge, avoiding the ship. One of the really nice things is that the design eliminates almost all moving parts due to its simplicity.