improver

from WordNet (r) 3.0 (2006)

improver
    n 1: someone devoted to the promotion of human welfare and to
         social reforms [syn: {humanitarian}, {do-gooder},
         {improver}]
    2: a component that is added to something to improve it; "the
       addition of a bathroom was a major improvement"; "the
       addition of cinnamon improved the flavor" [syn: {addition},
       {add-on}, {improver}]

from The Collaborative International Dictionary of English v.0.48

Improver \Im*prov"er\, n.
   One who, or that which, improves.
   [1913 Webster]