This is the J2-X. The liquid-fuel cryogenic rocket engine is the modernized version of the J-2, which NASA used in the late-'60s and early-'70s to thrust humans beyond low Earth orbit. (Click any image below for hi-res.)
Back then, NASA sent astronauts to deep space aboard the Saturn V. In the years to come, they'll be doing it with the Space Launch System. Perched atop the SLS will be the Orion Multi-Purpose Crew Vehicle — a manned spacecraft the Agency hopes will be used to deliver humans to the Moon (by 2021), asteroids and Mars.
The J-2X engines pictured here are being transported to the A-2 Test Stand at the Stennis Space Center in Mississippi, where engineers will run diagnostics on its performance capabilities, which are thought to exceed those of the J-2 by around 25%.
Orion is projected to make an unmanned test flight sometime next year.
Next year's Orion launch is (we hope) in preparation for an as-yet unannounced launch date for a return mission to the Moon.
All images via NASA; top image: Core components of the J-2X engine are installed on the A-1 Test Stand at NASA's Stennis Space Center in September 2007.