An early leader in proto-virtualization capabilities, Citrix was the company that popularized mainframe-like application virtualization in the client/server world. Its traditional products allow IT to run applications on a server where the data and code are controlled, letting business-unit workers use the application at their desks, viewing a remote session on their monitors and manipulating it using their own keyboards and mice.

After being left out of the buzz over virtualization for too long, and seeing the market shift toward VMware's approach, Citrix made itself VMware's only real rival by buying developer XenSource last fall and integrating its virtualization software with Citrix's own products. As the second-most recognized virtualization vendor, XenSource gave Citrix immediate credibility in the server-virtualization market to add to its reputation at the application-virtualization vendor of choice. It has since moved aggressively into desktop virtualization and has polished and packaged both virtualization-management consoles and its own end-to-end suite approach to server and application virtualization. Analysts say it might have the advantage over VMware when it comes to desktop virtualization, and has already put itself ahead of virtualization latecomer Microsoft by buying into the market so aggressively.

• XenServer
• XenDesktop
• XenApp
• NetScaler