It's a good fit any time you're considering storing a bunch of data in-memory for performance reasons. (Like telecom routing info.) Instead of writing a custom daemon, just spit it into VoltDB. Get replication, performance, etc. for free! Very neat.
Sadly the open source version isn't very ACID. They dropped the D from community edition. So you can scale out, but if any node dies, you're toast. There's still some uses, where you're running a transient or easily-rebuildable dataset. Or where you can manually run multiple full nodes (though I guess you'd need to implement cluster failover manually).
I guess it shows that it is hard to make a living off of open source products if they're really great. I've heard this from other open-source companies: the product's fantastic, no one pays. But make a taste as open source, basically a demo/trial, and get them to upgrade to commercial.