Lawrence Livermore National Laboratory - Livermore, Calif. (on-site, most require ability to get a security clearance)
Currently transitioning from MPI at massive (10^6 core) scale to MPI + ? Help us define the next generation of concurrency; will it be OpenMP, CUDA, Xeon Phi, nobody knows. We have developers working on everything from C/C++ physics simulator applications to research in computational techniques.
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[Speaking only for myself, and am not an official representative of LLNL.]