PetSC is a good example (both serial and MPI, as well as a large library of linear and non-linear solvers, included or presented as interfaces for external libraries). Trillinos is another example, but it is a much broader project, and not as well integrated as PetSC. Aztec has a number of solvers, as well as Hypre , which is hybrid (MPI + OpenMP).
All are based on MPI, at least in part; I don't know too many features with OpenMP support, although google offers Lis , which I am not familiar with.