We use Linux systems with a 2.4.9-7 kernel based on the KRUD 7.2 distribution, and 2.2.17-14 kernel based on the KRUD 7.0 distribution. We use our own software for parallelising applications but have experimented with PVM and MPI. In my view, the overhead for these pre-packaged programs is too high.
Linux is freely copiable.