I have compiled a survey ranking the most popular Virtual Appliances for Administration. It provides some insight into the types of applainces that are gaining traction in the Marketplace.
A dominant majority of the appliances seem to involve open source software on Linux, primarily because the software licensing considerations do not pose a barrier for the early adopters. The obvious side-effect of this is that there is not much revenue tied to this market today. Until the major software vendors, e.g., Microsoft, Oracle, SAP, etc., rationalize their licensing, the growth in unit deployments will be driven by open source stacks. Read my survey at