WordNet (r) 3.0 (2006):

eager beaver
    n 1: an alert and energetic person [syn: eager beaver, busy
         bee, live wire, sharpie, sharpy]

Moby Thesaurus II by Grady Ward, 1.0:

41 Moby Thesaurus words for "eager beaver":
   activist, addict, ball of fire, beaver, big-time operator, bug,
   bustler, busy bee, collector, demon, doer, energumen, enthusiast,
   faddist, fanatic, fiend, freak, go-getter, great one for, hobbyist,
   hound, human dynamo, hustler, infatuate, live wire, man of action,
   man of deeds, militant, new broom, nut, operator,
   political activist, powerhouse, pursuer, rhapsodist, sucker for,
   take-charge guy, visionary, wheeler-dealer, winner, zealot