One of the first attempts to build a powered exoskeleton was GE's 1965 Hardiman. Only one arm of the design was made to work and attempts at movement have been described as a mechanical version of St. Vitus Dance.
Although this suit could only be described as an utter failure, it lead other researchers' to continue work on powered exoskeletons.
Another interesting recent failure was Troy Hurtubise's Project Troy. The Canadian inventor of a suit designed to withstand grizzly attacks, modified his concepts to design the Trojan.
The suit is constructed from plastic (of the high-impact variety), with layers of ceramic for bullet protection, and ballistic foam. There's space for morphine, salt, a knife, light, pepper spray, and magnetic holsters for guns. The suit is supposedly even comfortable enough to sit for long periods. When the military failed to show interest in the suit, it was attempted to be auctioned off on Ebay, but failed to meet the reserve price.
Cyberdine in Japan is going to mass-produce Tsukuba University engineering professor Yoshiyuki Sankai's creation the Hybrid Assistive Limb (HAL).
According to Cyberdyne, the HAL allows someone who can normally lift 220 pounds on a leg press to hoist 396 pounds, an impressive 80% increase. While the device is primarily for civilian applications, such as use in hospitals and search-and-rescue work, I am sure military designers are taking note.
The XOS-system is being developed by Raytheon is one of the latest incarnations in design. While the unit has passed some impressive tests, major issues include the tether and lack of a portable power source.
Recently, the US Army has stepped forward with an additional $10 million in funding on the project.
With recent developments in Electroactive Polymers (EAP), the Pentagon is funding further research into these artificial muscles. These electrically-contractive fibers are intended to increase the strength-to-weight ratio of movement systems in military powered armor.
One day, within the next 20 years, this may be the vision of the common infantry soldier.