What a difference a year makes! When I first reviewed innotek’s VirtualBox, in version 1.3, I found a product with tremendous potential marred by some annoying stability and performance issues. Since then, the company has retooled VirtualBox, improving performance and reliability while also adding some compelling new features found nowhere else.

First, the particulars: VirtualBox is a cross-platform virtual machine management solution. It supports Windows (both 32- and 64-bit versions), Mac OS X, and various Linux distributions as hosts while providing a VMware-like laundry list of supported guest OSes. It maintains a consistent user interface across all of its host implementations, making it popular with users who multi-boot between Windows, Linux, and/or OS X. VirtualBox is available free for non-commercial use, and there’s even a GNU-licensed version for those who might want to tinker with the source code.

With version 1.5.0, VirtualBox introduced a major new capability: seamless windows. Basically, it’s a knock-off of the Coherence feature of Parallels Desktop for the Mac. The VirtualBox implementation, however, represents the first time the capability has been available under Windows or Linux. The latter should be of special interest to Windows defectors who have been looking for a reason to go the Linux route versus Apple. And though it’s not as well integrated with the Linux host as Parallels is with OS X (for example, you can’t create shortcuts or file associations between the host and guest), it’s free and integrates well with leading desktop distributions such as Ubuntu.

Other areas of improvement are mostly under the hood. For example, innotek now claims the capability to support many more concurrent virtual machines -- up to 96 instances of Windows XP on a dual-socket server with 32GB of RAM. Combined with VirtualBox’s support for headless operation (via the vboxsrvr process), this support allows innotek to position VirtualBox as a direct competitor to VMware Server and Microsoft Virtual Server in the server consolidation and desktop virtualization space.

I tested VirtualBox 1.5.2 on both Windows Vista Ultimate x64 and Ubuntu 7.10 “Gutsy Gibbon.” As with previous versions, installation was straightforward. The only caveat was the need to manually configure bridged networking support -- a process that involved creating a network bridge between the VirtualBox TAP adapter driver and the local NIC driver on the host. After this bridge was in place I could browse my local network from the guest OS and access resources on the host OS. As with previous versions, though, I did encounter the occasional IP address conflict message from both the host (Vista) and guest (Windows XP).