For a theoretically free operating system, Linux is -- and will continue to be -- a cash cow. A research firm sees a bright financial future for the OS.

IDC research recently unveiled new Linux usage and revenue figures putting current the OS's take just slightly under the $15 billion mark, with even more impressive numbers expected for the next four years, when overall revenue for servers, desktops, and packaged software running on Linux is estimated to reach $35.7 billion.

The numbers are higher than earlier estimates by most analysts, in part, said IDC, because it changed it methodology to account for not just Linux on new hardware, but also Linux that's redeployed on existing hardware, and even cases when the open-source OS is used as a guest operating system, such as in a server partitioned with virtualization software to run multiple OSes.