Also Included in the List: Ice-nine, Melange, Kryptonite and, uh, Supermanium and Atmosphereum.

Cheddite, n. The result of a college prank where a student placed a chunk of cheddar cheese in a powerful cyclotron. The resulting material was dense, black and produced a powerful energy field when exposed to electricity. This field could be used to flip a spacecraft through enormous distances virtually instantaneously when incorporated into a Cheddite drive unit.

Impervium, n. Material of which the doors of Scrooge McDuck's money bin are made, according to Carl Barks. Also, the material out of which darksteel is made. An element named impervium is also in the 1987 Teenage Mutant Ninja Turtles cartoon, used by Krang to power a force field generator.

[wikipedia: list of fictional elements, materials, isotopes and atomic particles]