THE FORBIDDEN ISLAND - good general overview site, by the residents and owners of the island.
Island of Niihau on polihale.com - lots of images and brief descriptions.
Ni'ihau - good, if brief, introduction.
Niihau. The American Heritage® Dictionary - includes a pronunciation.
Ni'ihau the Forbidden Island - pretty good overview.
Niihau in Hawaii - brief fact sheet.
Niihau Hotels and Resorts - not what the link says - actually a basic overview of life on the island, with info on their helicopter tours, as well.
Niihau island information - brief general overview.
Ni'ihau Lookout - basic info, plus images of the island from Kaua'i, with directions for getting to these lookouts.
Ni'ihau Medley - not sure where else to put this ... song lyrics.
Niihau Map - very basic info, plus a teeny-tiny map, in context with Kaua'i.
Niihau - Wikipedia - short article.