WordNet (r) 3.0 (2006):

buddy
    n 1: a close friend who accompanies his buddies in their
         activities [syn: buddy, brother, chum, crony,
         pal, sidekick]

Moby Thesaurus II by Grady Ward, 1.0:

134 Moby Thesaurus words for "buddy":
   ace, amigo, associate, aunt, auntie, bedfellow, bedmate,
   benchfellow, birthmate, blood brother, bosom buddy, boy, brethren,
   brother, bub, bubba, buck, bud, bunkie, bunkmate, butty, camarade,
   chamberfellow, chum, classmate, clubmate, co-worker, colleague,
   colt, comate, companion, company, compeer, comrade, confrere,
   consociate, consort, copartner, copemate, copesmate, couchmate,
   country cousin, cousin, cousin once removed, cousin twice removed,
   cradlemate, crony, cub, cupmate, daughter, father, fellow,
   fellow student, fellow worker, first cousin, fledgling,
   foster brother, frater, general partner, girl friend, gossip,
   grandnephew, grandniece, granduncle, great-aunt, great-uncle,
   half brother, hobbledehoy, jailmate, kid brother, lad, laddie,
   manchild, master, mate, messmate, mother, muchacho, nephew, niece,
   nuncle, nunks, nunky, old crony, pal, pard, pardner, partner,
   pewmate, playfellow, playmate, pup, puppy, roommate, running mate,
   schoolboy, schoolfellow, schoolmate, second cousin, secret partner,
   shelfmate, shipmate, shopmate, side partner, sidekick, sidekicker,
   silent partner, sis, sissy, sister, sister-german, sistern,
   sleeping partner, son, sonny, sonny boy, special partner,
   stepbrother, stepsister, tablemate, teammate, tentmate, unc, uncle,
   uncs, uterine brother, watchmate, waymate, whelp, workfellow,
   yokefellow, yokemate, young man, youth