Always wanted to play with your own Hyper V R2 Server but you did not want to install it on a disk? Or do you own a server with an internal USB connection and want to virtualize? Have a look at Hyper V R2, the free hypervisor of Microsoft. You can download it for free from Microsoft. The hard part however is to get it on a USB stick and make it bootable.
Microsoft has released some information about how to do this. A lot of steps, so read it quickly for the requirements and tips and go to the codeplex site to download a bit of tooling. This app will do all the steps described in the article.
BTW VMWare ESXI does also support booting from flash. You can use VMWare workstation to install the ISO to USB, see this post for details.
And while we are talking about booting from USB; UNetbootin can download and create bootable USB disks for you from existing Linux distributions.