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 multiboot between Windows, Linux, and/or OS X. VirtualBox is available free for noncommercial 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).

The default VirtualBox UI is clean and uncluttered, with most controls and functions easily accessible, and the "seamless windows" feature was remarkably easy to use. After the VirtualBox VM Additions were in place within the host OS, the new features menu option became available in the VirtualBox console window. Turn it on and the console window disappears, replaced by an overlay of the VM's Start Menu and Task Bar along the bottom of the host OS desktop. Launching an application from the Start Menu causes it to appear in a separate window -- one sporting the controls and scheme elements of the guest OS (for example, Windows XP buttons and scrollbars as opposed to the Aero, Aqua, or Gnome/KDE elements of the underlying host OS). In this sense, the integrated windowing is not exactly seamless, but it???s better than nothing. And for Windows/Linux users, it's the only show in town.

Unfortunately, VirtualBox 1.5.2 continues to be plagued by many of the same usability issues I noted in version 1.3. For example, you still can't drag-and-drop files between the host and guest OS. You have to use the Shared Folders mechanism, which also retains its complement of bugs from 1.3. On more than one occasion, I had the guest OS (Windows XP) crash while trying to access a folder on the host OS (Vista). Other times, Shared Folders didn't work at all, something I discovered was a common complaint on the VirtualBox support forums. And of course, the aforementioned bridged networking setup is still much too complicated compared to that of VMware and Virtual PC.

On the plus side, innotek seems to have worked out some of VirtualBox's performance kinks. Compared to version 1.3, the new version took roughly half the time to install Windows XP into a guest VM. Applications loaded faster within the VM and overall responsiveness was on par with VMware Workstation 6. The Snapshot feature, something I never got to work correctly in version 1.3, now seems to function properly. I was able to quickly roll back a VM to a previously saved state by simply selecting the Revert option during VM shutdown.

Overall, VirtualBox 1.5.2 is a solid update to a valiant effort by a tiny company (20 or so people) to compete with some very well-funded behemoths. The goals for VirtualBox continue to be ambitious, and as with the last iteration I tested, the realization of those goals remains just over the horizon. My advice to innotek: Slow down the development pace and spend a few weeks on shoring up what's already an impressive product, one worth a closer look by anyone seeking an alternative to the VMware hegemony.