Invisibility

[Fingolfin of the Noldor]

The chief power (of all the rings alike) was the prevention or slowing of decay, the preservation of what is desired or loved, or its semblance - this is more or less an Elvish motive. But also they enhanced the natural powers of a possessor - thus approaching 'magic', a motive easily corruptible into evil, a lust for domination. And finally they had other powers, more directly derived from Sauron: such as rendering invisible the material body, and making things of the invisible world visible. -letter 131 The Letters of JRR Tolkien

Here is a good FAQ which provides good quotes from the letters and other sources to common Tolkien questions including a bunch on the Rings of Power:

Scrolls of Orthanc

[ July 18, 2002: Message edited by: Fingolfin of the Noldor ]