What's interesting to me about Graph Engine
What you clearly want now is a 'database' which is scalable (throw more nodes at it when you need more space or speed), which stores things in RAM but properly persists them to (redundant) discs.
If you've got one of them, it's probable that the CPUs will be under-utilised (since they're mostly there to hang all the RAM off)
So the dream would be something that can do some compute on all those nodes. (Why copy data somewhere else for processing when there's an under-utilised CPU in the same place as the data)
That's what Graph Engine seems to be. And I think it's the only open source thing that ticks those boxes. OK, it's a graph database, but that probably fits real wold problems at least as well as SQL usually does when you've got your head round it.
What else can do that?