Solvers for elliptic partial differential equations are needed in a wide area of scientific applications. We will present a highly parallel CPU and GPU implementation of a conjugate gradient solver with an algebraic multigrid pre-conditioner in a package called Parallel Toolbox. The solvers operates on fully unstructured discretizations of the PDE. The algorithmic specialities are investigated with respect to many-core architectures and the code is applied to one current application. Benchmark results of computations on clusters of CPUs and GPUs will be presented. They will show that a linear equation system with 25million unknowns can be solved in about 1 second.