I have just committed a new thorn CactusUtils/NoMPI. This thorn contains a fake MPI implementation for much (but not everything) of MPI's functionality. It provides only a trivial implementation that runs on a single processor. NoMPI can be used to use an MPI code on systems where there is no MPI conveniently available, e.g. on laptops or workstations.
To use this thorn, add #include <nompi.h> to your source code, and "uses include header: nompi.h" to the interface.ccl. Thorn NoMPI will automatically switch itself off if a real MPI implementation is found, so that it can be safely used all the time.
My main goal was to make Carpet run on such systems, without having to add #ifdefs around each MPI call. Thorn NoMPI is not quite trivial, since certain functionality (e.g. MPI datatypes) still need to be supported.
-erik