There are some other operating systems that are free other than just Linux. One of the is the Linux/Windows hybrid (if you will). An example of this is: Cooperative Linux. This is an OS that runs both the OS's in their native state.
Also there are all the BSD OS's. BSD stands for: Berkley Software Distribution. The most popular of the BSD's is FreeBSD. You can find out information about it at http://www.freebsd.org
. Another popular BSD is PC-BSD. http://www.pcbsd.org
. For more BSD distros you can check out: http://www.bsd.org
For a list of a lot of Free/Open Source/Commercial OS's check out this website: http://www.freebyte.com/operatingsystems/