To install Open MPI as described in this HOWTO, you'll need the following: Almost any machine purchased in the last year or two has multiple processor cores. Second, multi-core computers are the future.
To me, this is the biggest motivation, since I can test software more easily on my desktop (or laptop) machine than on a cluster. Why might you consider installing Open MPI? First, many programs that were designed to run distributed across several computers using MPI can now be run on a multi-core modern desktop computer.