The p4 package, now no longer supported, was popular a few years ago. It does message-passing and shared-memory operations (the latter are supposed to be for shared-memory hardware but some work even on networks of workstations).

Information on p4:

• The package is available at the p4 home page, and at my mirror site.

• For a quick introduction, see notes by Prof. Matloff.

• Here is the official documentation.