The VASIMIR VF-200 is an honest-to-goodness plasma thruster that NASA has just agreed to test out in Earth orbit, potentially aboard the ISS. If things work out, it's capable of taking us to Mars in 40 days instead of six months.
VASIMIR stands for "Variable Specific Impulse Magnetoplasma Rocket," but the bit that you should really care about, besides "rocket," is "magnetoplasma." VASIMIR works, as all rocket engines do, by firing energetic (i.e. hot and/or fast moving) stuff out its backside, which generates force in the opposite direction. In the case of VASIMIR, that stuff is an ionized plasma created by heating up argon gas using radio waves powered by electricity. A magnetic field shields the engine while directing the plasma out the back at a staggering 50 km/s, and the whole package weighs in at only 300 kg with a fuel efficiency of 60%, which is something like three times better than the space shuttle.
These are all great numbers, but one of the most important ones is the amount of thrust produced by VASIMIR: the 200 kilowatt engine can lift about half a kilogram. Now, okay, that's barely anything, but the engine is so fuel efficient that unlike conventional propulsion systems you can just turn it on and leave it on, and even that small amount of thrust can build up velocity very quickly, enabling a trip to Mars in 40 days or extremely efficient (but slower) cargo hauling to lunar orbit.
The immediate plan is for a pair of VASIMIR thrusters combined into one single engine to be (possibly) attached to a dedicated vehicle in Earth orbit or (probably) attached to the ISS. The solar panels on the ISS would be about to provide enough electricity to test fire the engine at full power. Somewhat ironically, part of the reason that the ISS would directly benefit from VASIMIR is that those solar panels create a substantial amount of drag, which slows down the ISS enough that it has to rely on fuel-guzzling thrusters (or help from the space shuttle) to give it a boost back to a higher orbit every now and then.
Now that VASIMIR has made that critical jump from a testing bench here on Earth out into space, it seems destined for interplanetary exploration, enabling transit times that are short enough for it to make sense to send humans instead of just robots.