D-n quivers from branes

D-branes can end on orbifold planes if the action of the orbifold group inc ludes (-1) (FL). We consider configurations of D-branes ending on such orbi folds and study the low-energy theory on their worldvolume. We apply our re sults to gauge theories with eight supercharges in three and four dimension s. We explain how mirror symmetry for N = 4 d = 3 gauge theories with gauge group Sp(k) and matter in the antisymmetric tensor and fundamental represe ntations follows from S-duality of IIB string theory. We argue that some of these theories have hidden Fayet-Iliopoulos deformations, not visible clas sically. We also study a class of finite N = 2 d = 4 theories (so-called D- n quiver theories) and find their exact solution. The integrable model corr esponding to the exact solution is a Hitchin system on an orbifold Riemann surface. We also give a simple derivation of the S-duality group of these t heories based on their relationship to SO(2n) instantons on R-2 x T-2.