AWS recently announced support for nested virtual machines within virtualized EC2 instances running KVM or Hyper-V. A ...
If you need to use virtual machines on Linux, try Virt-Manager. Virt-Manager is more reliable than VirtualBox. Virt-Manager is completely free and open-source. I'm a long-time supporter of VirtualBox.
Besides their utility in self-hosted projects, Proxmox LXCs are just as viable for general server experiments ...
A simple VM manager with bash backend that runs on flatfile databases. It requires no setup (except setting properties in the config.php script). SVMM is released under the CC-BY-NC-SA v3.0 unported.