VirtualBox 4.0.0 (released 2010-12-22)

VirtualBox 4.0.0 is a major update and includes the following major new features:

• Reorganization of VirtualBox into a base package and Extension Packs
• New settings/disk file layout for VM portability
• Major rework of the GUI (now called “VirtualBox Manager”):
• Redesigned user interface with guest window preview (also for screenshots)
• New virtual hardware Intel ICH9 chipset with three PCI buses, PCI Express and Message Signaled Interrupts (MSI)
• Intel HD Audio, for better support of modern guests

According to LifeHacker you can even use VirtualBox to run a Mac OS-X guest !