libNEGF is a general library for Non Equilibrium Green's Functions.
With libNEGF you can calculate Equilibrium and Non Equilibrium Green's Function in open systems and related quantities, within an efficient sparse iterative scheme.
libNEGF is available under LGPL license.
For an example of application, see the use of libNEGF in dftb+
*A. Pecchia, L. Salvucci, G. Penazzi and A. Di Carlo, New Journal of Physics 10, 065022 (2008)