Virtualbox is geek manna: a bunch of computers in one. As long as you have enough memory, LOL.
Version 4.0 was just released. Here are some resources.
Download: Oracle VM VirtualBox 4.0
Download VirtualBox 4.0 for Solaris 10 5/08 and later, 32- and 64-bit versions of Linux, Windows, and MacOS. Freely available. No license required. Note the license requirements for the operating systems you install on VirtualBox, however.
Blog: What's New in Version 4.0
The Fat Bloke describes what's new in Version 4.0, including the revamped VirtualBox Manager and other usability improvements, increased throughput, virtual appliance sharing, support for new virtual hardware, and more.
OTN Live video: VirtualBox and Oracle's Virtualization Story
Wim Coekaerts describes how the combination of Oracle's Virtual Desktop Infrastructure, VirtualBox, and Sun Ray systems provide a complete desktop to datacenter virtualization story.
VirtualBox 4.0 User Guide
How to intall, configure, and use VirtualBox 4.0. How to manage virtual storage, networking, and remote virtual machines. API's, troubleshooting, and security.
Blog: Changes to Configuration Files In Version 4.0
The Fat Bloke describes how version 4.0 configuration files have changed, including where virtual disk and snapshot files are saved.
The VirtualBox Community
Virtualbox.org is the VirtualBox community, where you can download source code, contribute code, read documentation, and get the latest news about VirtualBox.