Run it bare metal on a machine (like Cisco made-for-this blade boxes), with a SAN for disk, and it's a sight to behold. Minutes to provision new instances (as opposed to weeks waiting for new hardware to come in), near instant failover, etc.

KVM for Linux, xxx for Solaris (I don't remember the name), HyperV for Windows... but you can run all of that in VMWare. Even Sparc Solaris for that matter.

But that's in an enterprise data center that needs that sort of thing.