Well, just look on the box top and... uh no, seriously, there are a bunch of different distributions of Linux out there & they're all pretty much free (as in "speech" AND as in "beer"). Linux is Open Source Software, which means the source code is free to anyone who wants it, to build anything they like with it so long as the code for whatever they build is also open source. So the one I use now is Ubuntu but there are several spinoffs from that one, including Kubuntu, which mostly just uses a different desktop. I used to use SuSE Linux a few years ago, and one called Simply Mepis, and another called Knoppix, & a few others I can't remember. The thing is, originally Linux was based on the UNIX operating system which was (and is) heavily used in business applications. In those days it was run entirely from the command line - no graphical user interface at all. And everything it is now, everything it can do, is the result of a huge bunch of Global community projects. Mostly just people pitching in to help make it work, devoting their time, knowledge & labor to get it done for everyone.
I'm guessing you're using Ubuntu like I am because it's currently the most popular & most user-friendly distro out there AFAIK. I *think* it's the only one you can download & install without having to partition your hard drive to make a space especially formatted for it. Instead, it will install & run on a Windows machine on its "C" drive. That might be the biggest step yet toward making Linux a more "mainstream" operating system acceptable to the Win-dozing public!