In this talk, we discuss the Shintani double zeta functions, which were introduced by Shintani to study the zeta functions associated with the space of binary quadratic forms. First, we give an explicit formula of the Shintani double zeta functions. Next, as its applications, we derive new functional equatons, and prove an asymptotic formula of the weighted average of the central values of quadratic Dirichlet L-functions. This is a joint work with Henry Kim and Masao Tsuzuki.