The Portable, Extensible Toolkit for Scientific Compu- tation (PETSc) is a suite of data structures and routines that provide the building blocks for the implemen- tation of large-scale application codes on parallel (and serial) computers. PETSc uses the MPI standard for all message-passing communication.

http://www.mcs.anl.gov/petsc/petsc-current/docs/

PETSc users manual (pdf)