Portable message passing systems,
This is the new international standard and is expected to become much more prominent in the coming years. It is very similar to PVM but has a little extra functionality.
This is currently (1996 Jun) the most popular.
This was Europes' attempt at a message passing system. It was very successful and provided a lot of input to the MPI project.
All systems can operate on heterogeneous networks.
For example, (pseudo code),
    Send(to_pid,buffer,length)   ! Send buffer
    Recv(from_pid,buffer,length) ! Receive buffer
    Bcst(from_pid,buffer,length) ! Broadcast buffer
    Barrier()                    ! Synchronise