I ran an OpenMOSIX cluster as a hobby. The alternative was Beowulf (the meme of the day was "Imagine a Beowulf cluster of these things").
It seems the mainstream server industry has moved to more isolation rather than more interconnectedness, which is probably better for most public-facing systems.
Isolation is orthogonal to what I listed. MOSIX has decent sandboxing. I don't know about Linux-PMI or OpenMOSIX, though. They died off years ago anyway.
