OpenVZ virtualization is an OS level container-based virtualization, and it has resources that are divided between users on a physical server. Each container acts like a stand-alone virtual server and can be accessed with a root (SSH) connection. As a separate server container can be rebooted separately, it also has a dedicated IP address, shared RAM, individual processes, files, applications, system libraries and configuration files. On OpenVZ, the kernel cannot be modified. It has its stable version and modules cannot be added. The good thing regarding this virtualization is a faster performance, and a lower need of resources.
For free, you'll be getting one micro instance, 5GB of Cloud storage. There are 15 total Google services which you'll be able to run without paying anything. This VPS is made for people who want to build. It's not the place if you wish to make a WordPress blog. But it's ideal for those who want to use Google's vast resources to make something brilliant.
The golden rule for unmanaged VPS hosting is this: If you have a problem with your unmanaged VPS Account, it is your problem. If the problem is software related, resource related, performance related, need troubleshooting or configuration of software, or need general help understanding Unix or server applications, you are on your own and should not expect help from the web host.
koding.com has a free VM running Ubuntu. The specs are pretty good, 1 gig memory for example. They have a terminal online you can access through their website, or use SSH. The VM will go to sleep approximately 20 minutes after you log out. The reason is to discourage users from running live production code on the VM. The VM resides behind a proxy. Running web servers that only speak HTTP (port 80) should work just fine, but I think you'll get into a lot of trouble whenever you want to work directly with other ports. Many mind-like alternatives offer similar setups. Good luck!
A couple of years ago I remembered going to a demo and watching Hyper-V crash, but since then a lot has changed. Hyper-V can now do many of the same things most enterprise virtualization software boast about. Live migration, HA, templates, and importing VMs from VMware and EC2 using SystemCenter VMM. The only real hang up is the lack of popularity with the Linux community