Sun Microsystems has signed a stock purchase agreement to acquire Innotek GmbH, a Weinstadt, Germany-based supplier of VirtualBox virtualization software.

VirtualBox is open source code aimed at developers who want one or more virtual environments that match the target environment for their prospective application. Innotek is the company established to support VirtualBox and manage its development.

Sun said VirtualBox has been downloaded more than 4 million times since being made available in January 2007, and Sun moved quickly to become the acquirer as it maps out a future suite to virtualize customer environments. It plans to use VirtualBox to extend the Sun xVM virtualization software, its hypervisor based on open source Xen.

VirtualBox allows a desktop or laptop computers running Windows, Linux, Solaris, or Mac OS to run additional operating systems side by side, switching between them with a mouse click. Software writers can use the capability to produce cross-platform applications. Regular users can choose to run apps that would not normally be available under their built-in operating system.

VirtualBox may be downloaded at VirtualBox.org. Its footprint is less than 20 MB, Sun officials said. But it's primarily aimed at developers and Linux distributors, not general-purpose users, a warning on its Web site states. There's no automated installer or setup utilities for the download.

VirtualBox can run any version of Windows from 3.1 to Vista; it runs Linux powered by the 2.2, 2.4, or 2.6 kernels. It runs Solaris for x86, OS/2, Netware, and Microsoft DOS.

Sun said its acquisition will give VirtualBox worldwide technical support along with Sun's worldwide sales force and network of partners to promote its use. Terms of the acquisition were not revealed because the purchase price "is not material to Sun's earnings per share," the announcement said.

Surgient, VMware, VMLogix, and Virtual Iron all offer virtualized environments on servers for testing new software. VirtualBox is a similar idea positioned closer to the developer on his own machine.