[1] : The Collaborative International Dictionary of English v.0.48

doer \do"er\, n. from do, v. t.  i.
   1. one who does; one who performs or executes; one who is
      wont and ready to act; an actor; an agent.
      1913 webster

            the doers of the law shall be justified. -- rom. ii.
                                                  13.
      1913 webster

   2. scots law an agent or attorney; a factor. --burrill.
      1913 webster
see also:
do 

[2] : WordNet (r) 2.0

doer
     n : a person who acts and gets things done; "he's a principal
         actor in this affair"; "when you want something done get
         a doer"; "he's a miracle worker" syn: actor, worker
see also:
actor worker 

[3] : Moby Thesaurus II by Grady Ward, 1.0

22 moby thesaurus words for "doer":
   activist, ball of fire, beaver, big-time operator, bustler,
   busy bee, eager beaver, enthusiast, go-getter, human dynamo,
   hustler, live wire, man of action, man of deeds, militant,
   new broom, operator, political activist, powerhouse,
   take-charge guy, wheeler-dealer, winner

Doer >> D Words
Doer, definition of term: Doer
doer_pag1.html