ARGOS stands for "Active Response Gravity Offload System." It works by using a computer controlled overhead crane and a harness that soak up a portion of your weight, depending on what level of gravity you want to simulate. The way that it differs from other, similar systems is that the computer makes sure that it's only offloading force in the vertical direction: unlike a more traditional cable and harness system, you're not feeling any sideways tugs as you move around. Check out the video below, and watch the crane constantly adapting its position based on the movements of the user.
ARGOS isn't quite finished, but when it is, it'll be capable of handling everything from space suits to robots, all the way from Mars gravity down to the microgravity that we'll need to practice in for our first visit to an asteroid.