VMware said Tuesday it will launch a Virtual Appliance Marketplace to capitalize on the growing interest in downloading software as an application ready to run with its own optimized operating system.

Software appliances in the past have included firewalls and other stand-alone applications that function best when an operating system has been configured to run with them. With virtual appliances, the two [application and OS] are packaged together in a single file that has been formatted to run in a virtual machine. "Server consolidation has driven virtualization so far," noted Diane Greene, president of VMware in a keynote address at the company's annual user conference in Los Angeles, but virtual appliances "represent a transformation in how we build and deliver software."

VMware, with its VMware Server and ESX hypervisor virtualization software, is both the virtualization market leader and the virtual appliance leader, with software companies gravitating to VMware as the foundation for new appliances. Over the last six months, the list of software appliances available for download from its Web site has increased from near zero to 330.

Most of the virtual appliances are based on Linux. But Microsoft has noted the trend toward appliances. It announced the day before VMworld opened that both it and 20 of its technology partners would start distributing Windows 2003 and SQL Server 2005 in virtual hard drive format to give customers a faster means of testing and evaluating its products. Microsoft's partners add their applications to the bundle, which ship ready to run in Microsoft Virtual Server.

When a customer gets a virtual appliance, evaluating it becomes a matter of moving a computer file to a server with an available virtual machine, a task that might take a few minutes. Previously, enterprise software evaluators needed to order a server and get the software installed on it, a process that took from several hours to several days.

"We believe this is how most software will be deployed" in the future, said Raghu Raghuran, VMware's VP of product management in an interview.

"Downloading applications becomes as easy as downloading iTunes," said Pat Kerpan, CTO of CohesiveFT, a maker of business-specific software appliances.