The Helicarrier is a flying aircraft carrier that acts as a mobile HQ for SHIELD. It is powered by a fusion core, and partially supported by "repeller rays". At least six have been seen, with variants used by other organizations. They cost at least 9 billion dollars each.
In Mighty Avengers #1, Iron Man introduced Helicarrier Gold, basically a gold Helicarrier. Sadly, it was recently destroyed by Red Hulk in Hulk #2.
In "Blade", the Vampire-hunting SHIELD-offshoot Silvereye used a custom carrier called Blackeye-1, with a silver-impregnated hull and a cloud-generator for stealth. Destroyed in Blade: Vampire Hunter #6.
In Livewires, the mysterious villain codenamed "White Whale" (a hive-mind of Nick Fury LMDs) was stationed on a Helicarrier that had been recovered from wreckage and repaired.
The Aeromarine is a cluster of submarines with rocket engines bolted together, used in Nextwave. It is armed with Drop Bears and a squadron of H.A.T.E Pterosuits, Samuroid Batch 23, the very phallic-looking Armageddon Horn, and a lot of big @%^$# guns.
Ultimate S.H.I.E.L.D uses a fleet of airborne carriers, retrofitted from Naval sea vessels using a special rocket engine designed by Tony Stark. Most were destroyed by the Chitauri in Micronesia. They were also given a Helicarrier by Ultimate Galactus.
Colonel Fury had a Helicarrier commissioned and built by Tucker Ford, the Biogeneral. Fury is using it as the mobile headquarters of the new Squadron Supreme.
In a desperate attempt to save mutants and humans alike, Nick Fury summons all uninfected superheroes to the S.H.I.E.L.D Helicarrier to develop a cure and rescue as many humans as they can. During one of the evacuations, nearly all of the uninfected guarding the Helicarrier lead by Magneto become casualties by being eaten whole, or have been infected.
It appeared in The Avengers and at first it can float in the sea as an aircraft carrier.