  Effective field theories in type I and II superstring theories for D-branes
located at points in the orbifold C^2/Z_n are supersymmetric gauge theories
whose field content is conveniently summarized by a `quiver diagram,' and whose
Lagrangian includes non-metric couplings to the orbifold moduli: in particular,
twisted sector moduli couple as Fayet-Iliopoulos terms in the gauge theory.
These theories describe D-branes on resolved ALE spaces. Their spaces of vacua
are moduli spaces of smooth ALE metrics and Yang-Mills instantons, whose
metrics are explicitly computable. For U(N) instantons, the construction
exactly reproduces results of Kronheimer and Nakajima.
