We use VirtualBox for some of our virtual servers. The command line flexibility and compatibility with RDP makes this a very useful option.
However, it does take some digging at times to resolve issues.
The latest issue for us was running VMs in Headless mode i.e. without any graphical UI. VirtualBox then uses RDP (Remote Desktop Protocol) which means you can view any running virtual server with the standard Microsoft RDP viewer (mstsc.exe).
However, in the latest version of VirtualBox Oracle have moved the RDP functionality out of the core server. So you have to install a separate extension pack.
Thanks hugely to HowToForge
"Starting with version 4.0, VirtualBox has introduced so called "extension packs" and has outsourced some functionality like remote desktop connection support (VRDP) that was part of VirtualBox packages before version 4.0 into these extension packs. Because we need remote desktop connections to control our virtual machines, we need to install the appropriate extension pack now. Go to http://www.virtualbox.org/wiki/Downloads, and you will find a link to the following extension pack:
VirtualBox 4.0 Oracle VM VirtualBox Extension Pack
Support for USB 2.0 devices, VirtualBox RDP and PXE boot for Intel cards
Download and install the extension pack as follows:
VBoxManage extpack install Oracle_VM_VirtualBox_Extension_Pack-4.0.0-69151.vbox-extpack"