As a technical term
"Niggerhead" was, among others:
a former British term for a black iron post for mooring ships, made from an old cannon partially buried muzzle upward, with a slightly oversize black cannonball covering the hole
a former sailors' term for an isolated coral head, notorious as navigation hazards
among some American stonemasons, a term for a large smoothly rounded stone
an antiquated term for a large round rock sticking up from the surface of a logging road, used by loggers and log truck drivers in the Pacific Northwest
an old U.S. Navy term for a small winch, a Capstan
a coal miner's term for blackish iron disulfide nodular rock occurring at the top of the coal seams, often visible in coal mine roofs. Because of their density, they can fall from the roofs, injuring or killing the miners below
an archaic term for the striking weight on a pile driver
a term for a steam manifold, fountain, turret or header on a boiler, particularly that of a locomotive
Plants and animals
"Niggerhead" is or was a common name for:
Carex secta, a tussock grass in New Zealand
Echinocactus polycephalus, the niggerhead cactus, native to California and Arizona
Echinacea, or coneflower, colloquially called "Kansas niggerhead" or "wild niggerhead"
Nasutitermes graveolus, the niggerhead termite, native to Australia
Rudbeckia hirta, a flowering plant in the family Asteraceae
hard tussocks in the tundra, esp. in Alaska
This list is incomplete; you can help by expanding it.
"Niggerhead" is part of the name of:
Nigger Head, an island in North Queensland, Australia
Niggerhead hunting camp, the former name of a hunting camp of Texas governor Rick Perry, renamed to "North Camp Pasture" in about 2006 despite the camp being in the posession of the Perry family since the early 1980's.