Lets start the answer with an explanation of what Linux is. Linux is only a...
Lets start the answer with an explanation of what Linux is. Linux is only a kernel which is a set of drivers and libraries used to allow hardware and software to communicate with hardware on computer based systems.
The Linux kernel is used in:
* Person Computer systems running Linux based distributions (desktops, laptops and netbooks)
* Servers systems running Linux based distributions
* Networking hardware systems running custom Linux based distributions (routers, switches, and wireless devices)
* Cell phone systems running Linux based distributions (WebOS, Android and Meego)
* Home Entertainment Hardware such as DVRs, DVD players and Televisions
* Personal Navigation Systems such as Tom Tom and Garmin
* Personal Media Players
* Digital Cameras
* Custom Hardware and Embedded systems used by companies around the world for various tasks like embedded robotic systems
Most likely you were referring to Linux based distributions in the original question and in that case the answer is simple, it is used by companies, individuals, non-profit organizations, governments, etc.... it is used anywhere that you can also find windows or apple computers.
Because of the opensource and free nature of the Linux kernel its future is wide open, anyone who would like to minimize the development and implementation time and costs of a computer based system can use it. This means anywhere that there is a need for the Linux kernel it can be implemented, that leave the future uses limited only by our imagination. I personally can't wait to see what comes to the world in 5 years using the Linux kernel.