The Collaborative International Dictionary of English v.0.48:

Brownie \Brown"ie\, n. [So called from its supposed tawny or
   swarthy color.]
   An imaginary good-natured spirit, who was supposed often to
   perform important services around the house by night, such as
   thrashing, churning, sweeping. [Scot.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

Brownie
    n 1: a junior Girl Scout
    2: (folklore) fairies that are somewhat mischievous [syn: elf,
       hob, gremlin, pixie, pixy, brownie, imp]
    3: square or bar of very rich chocolate cake usually with nuts

Moby Thesaurus II by Grady Ward, 1.0:

95 Moby Thesaurus words for "brownie":
   Ariel, Befind, Corrigan, Finnbeara, Lilliputian, Mab, Oberon,
   Titania, Tom Thumb, apple-polisher, ass-licker, backscratcher,
   backslapper, banshee, biscuit, bootlick, bootlicker, brown-nose,
   clawback, cluricaune, cookie, courtier, creature, cringer,
   date bar, dupe, dwarf, elf, fairy, fairy queen, fawner, fay,
   flatterer, flunky, footlicker, fruit bar, ginger snap,
   gingerbread man, gnome, goblin, gremlin, groveler, handshaker,
   helot, hob, homunculus, imp, instrument, jackal, kobold, kowtower,
   lackey, ladyfinger, led captain, leprechaun, lickspit, lickspittle,
   macaroon, manikin, mealymouth, midge, midget, minion, ouphe,
   peewee, peon, peri, pip-squeak, pixie, pooka, puca, puppet, pwca,
   pygmy, runt, serf, shortbread, shrimp, slave, spaniel, sprite,
   stooge, suck, sugar cookie, sycophant, sylph, sylphid, timeserver,
   toad, toady, tool, truckler, tufthunter, wart, yes-man