The Collaborative International Dictionary of English v.0.48:

nigger \nig"ger\, n.
   A negro; -- in vulgar derision or depreciation. It is usually
   intended and interpreted as highly insulting and vulgar.
   [Low, deprecatory, and vulgar]
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

nigger
    n 1: (ethnic slur) extremely offensive name for a Black person;
         "only a Black can call another Black a nigga" [syn:
         nigger, nigga, spade, coon, jigaboo, nigra]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "nigger":
   American Indian, Amerind, Australian aborigine, Bushman, Caucasian,
   Indian, Malayan, Mister Charley, Mongolian, Negrillo, Negrito,
   Negro, Oriental, Red Indian, WASP, black, black man, blackfellow,
   boy, brown man, burrhead, colored person, coon, darky, gook, honky,
   jigaboo, jungle bunny, niggra, ofay, paleface, pygmy, red man,
   redskin, slant-eye, spade, the Man, white, white man, whitey,
   yellow man