VirtualBox - a special program to create a memory PC virtual machines. Each virtual machine can contain an arbitrary set of virtual devices and a separate operating system.
Scope of virtual machines is very wide - from performing the functions of software testing to create entire networks, which are easily scaled, load-balance and protect. VirtualBox is free, open source software.
ChangeLog 4.1.8
This is a maintenance release. The following items were fixed and / or added:

VMM: fixed VERR_MAP_FAILED during savestate under certain circumstances (bug # 7929)
GUI: stop updating the VM status icons when the VM is paused (bug # 8368)
VBoxManage: fixed wrong return code after startvm (bug # 9642)
BIOS: fixed hang at launch of DOS applications generated by Clipper 5.3 (note that hardware virtualization may be required)
USB: fixed OS / 2 boot hang when using recent USB drivers
NAT: increase maximum number of parallel connections making connections with port forwarding more robust (# 8471)
Metrics: fixed potential problem with invalid access in guest metrics collection upon VM destruction
Main: don't crash if a medium is ejected twice (bug # 9922)
VBoxSVC: fixed crash under rare circumstances (eg client crash)
VRDP: fixed screen freeze (bug # 9620)
OVF / OVA: fixed broken disk images on import under rare circumstances
OVF / OVA: better error message when importing corrupted appliances
VMDK / VHD: fixed a possible corruption with host cache disabled when using snapshots under rare circumstances (bug # 9749)
3D Support: Fixed fullscreen mode issues for ATI graphics (bug # 9775), Windows Media Player rendering for XPDM-based Direct3D support (bug # 8341). Multiple fixes to XPDM and WDDM - based 3D support for Windows Guests and for 3D support in general
Linux hosts: fixes for Fedoras Linux 2.6.41 (bug # 9948)
Linux hosts / guests: fixes for Linux 3.2 (bug # 9743)
Solaris Additions: various shared folder fixes (bugs # 9856, # 9862, # 9917)
Windows Additions: various fixes for Direct3D support (un) installation, added detection of missing or wrong Direct3D system files