From The Collaborative International Dictionary of English v.0.48 :
Improver \Im*prov"er\, n.
One who, or that which, improves.
[1913 Webster]
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]