I use Xen, with a Debian Dom0. You can provision a new VM in about a minute. The commandline tools are not very obvious, and they have scores of subcommands. B

I use Xen, with a Debian Dom0. You can provision a new VM in about a minute. The commandline tools are not very obvious, and they have scores of subcommands. But it's all scriptable, so you can easily have a CI system spin-up and tear-down VMs. You can hot-copy a VM from one physical machine to another, which is neat for maintenance. It's super-lightweight, but I'd say Xen is a bit nerdy for many people.