I'm obviously not the parent commenter and have no inside knowledge of the HPC world, but my guess is the main open source supercompting projects would be from NASA, the US national labs, and CERN.

You might search HN for the recent announcements about new clusters, especially top500, and look for the comments discussing using MPI (versus something custom, I think?), as my recollection is that those topics would yield further pointers to the actual examples you're looking for.

Take a look at parallel simulation codes (for example, LAMMPS in chemical physics).