Russian site Spidermedia has posted concept art from Joss Whedon's The Avengers, which features S.H.I.E.L.D.'s infamous Helicarrier, as visualized by artist George Hull:

Early sketch of the Helicarrier design #1
Early sketch of the Helicarrier design #2
RELATED: Hitman Director Said No to Marvel Fearing Heavy Studio Interference

The S.H.I.E.L.D. Helicarrier was originally supposed to appear in Jon Favreau's Iron Man 2. It is an aircraft carrier specifically designed to be capable of independent powered flight, in addition to the conventional functions of aircraft carriers. It is the signature capital ship of the intelligence/defense agency S.H.I.E.L.D. According to Spidermedia, Kevin Feige plans to build a life-size replica of the Helicarrier to display atop the convention center at Comic-Con 2011.

B. Alan Orange at Movieweb
B. Alan Orange