Proxmox VE is a bare metal hypervisor product that uses KVM and OpenVZ virtualization technologies. It uses Debian Linux as its base operating system and unlike a lot of other similar projects out there actually works well with no major gotchas. It's actually so well executed that I consider it a viable alternative to VMware ESXi.
Proxmox VE requires a machine with a 64 bit CPU and VT flags in the bios in order to do full KVM based VM's. If you lack the VT flag you can use the openVZ based technology that is very similar to BSD Jails or Solaris Zones. There are some limitations with the openVZ VM's but the upside is you can run them on a cheaper older hardware that lacks VT flags such as a Dell Poweredge 1850. Proxmox VE includes a number of operating system templates that allow you to provide most popular distributions of linux quickly and easily by downloading additional template files. If you want to take advantage of some of the Live Migration (aka vmotion) technologies shared storage is helpful but not required. Some of the OpenVZ vm's are able to move from host to host easily because they are not based on large disk images. Even if you don't utilize the live migration features VM's can be moved back and fort between systems through the web interface and without resorting to painfully slow SCP.
VM provisioning is handled through a rather well done web interface. OpenVZ vm's can be booted and installed in under 10 seconds with all configuration such as IP assignment and root passwords handled from the web interface. KVM VM's do require OS installation and manual configuration due to the style of virtualization but work very well. The Console is handled over VNC and works well even over slow network connections.
One of the big selling points of VMware ESXi to many people is the CPU and memory overcommit capabilities. With a simple Kernel upgrade Proxmox VE offers a similar feature set and impressively runs Windows 2008 R2 better than VMware ESXi 4.0.