You know, an awful lot of people never even think about the operating system they use. They just use the computer as a tool to get the job done. That's fine, of course. The very fact that people can use a computer without having to worry about what the operating system is doing and in a simple and readily-understood way is definitely progress.
Nowadays, you can generally buy a new computer, unbox it and be online in a matter of minutes, with very little assistance. That means the technical barrier to computer use is very low - I mean, plenty of elderly people use computers, as do quite young children.
By comparison, in the late 1980's when I got my first computer (an Amstrad CPC 6128) it was nothing like as easy to use. It had a command-line interface only, and no mouse. Every manufacturer installed their own operating system (often on a ROM, not a hard drive, so there was absolutely no prospect of being able to change it). There were some great OS's available (apparently the Commodore Amiga had a great OS, for one), but many of them required a great deal more technical ability to use than is now required.
Nowadays, the pendulum has swung in completely the opposite direction. With a few minor exceptions, all computers come with either Windows or Mac OS X installed. While both these OS's are quite easy to use, to me it seems that two operating systems can't possibly be sufficient to be the best OS for everyone in the world. I wouldn't want to return to the chaotic mix of OS's available in the late 80s and early 90s, but I do think that a little more diversity does a lot of good. After all, if Microsoft and Apple had more effective opposition, they'd improve their products to compete, so end users benefit.
Linux, of course, is far and away the most popular alternative OS in all its various incarnations, and is beginning to make its presence felt as a preinstalled OS as well, thanks to products like the gPC, Dell's PC's with preinstalled Ubuntu, and the Asus Eee PC. But, it's certainly not the only one. There's plenty of other alternatives. And it's worth looking into some of these if you're in the market for a new operating system.
FreeBSD is based on BSD UNIX and has actually been around for longer than Linux. Unlike Linux, it's managed as one project (as opposed to Linux, which is actually a number of different projects that create different elements which are then put together to create a Linux distro such as Ubuntu or Fedora), giving the whole thing a more consistent feel. It's used on servers a lot, but can also make a fine desktop OS. However, it's harder to use than a Linux distribution on its own (unless you use a tough one like Slackware). Fortunately, a number of projects exist to turn FreeBSD into a more easily useable desktop OS, such as Desktop BSD and PC-BSD, so these make a great alternative for beginners. FreeBSD makes for a stable and powerful operating system.
There are other BSD variants as well. Two notable examples are OpenBSD, which is the most secure operating system in existence, and NetBSD, which is designed to be highly portable and capable of running on many platforms.
OpenSolaris is one that could potentially grow to rival Linux. Like Linux, it is an open source Unix-based operating system. However, at present it is generally harder to use than Linux and hasn't got the kind of huge community behind it that Linux has. That said, OpenSolaris distributions are beginning to appear - BeleniX will work as a Live CD, and Project Indiana aims to build a complete distribution that makes it easy to use OpenSolaris as your main OS (they've even hired Ian Murdock, creator of the Debian Linux distribution, to work on the project). However, although the Linux and OpenSolaris kernels are broadly compatible from a technical point of view so drivers could be easily ported across, due to incompatibilities between the licenses, it's not possible to include these drivers in OpenSolaris, so hardware support lags behind that in Linux.
Another OS that could potentially become available in the future is GNU/Hurd. If you know much about Linux, you'll know that there are essentially two parts to it - the Linux kernel and the tools it uses. These were created by two separate projects - the GNU project created the tools, while Linus Torvalds created the Linux kernel. However, the GNU project had always intended to create their own free operating system from scratch, so after they had created the tools they needed, they began work on an advanced kernel of their own, called the Hurd. Progress on this was slow, however, and eventually in the early 1990's the Linux kernel became available, and this was compatible with the GNU tools (this is why Linux is correctly referred to as GNU/Linux, because it was created by GNU and the Linux project). It's still in development (and has been for a very long time!), but the Debian Project have succeeded in porting Debian to the Hurd kernel, and it's possible it may one day be available as an alternative to Linux (but don't hold your breath!).
Minix is less mature than either Linux or FreeBSD, but is smaller. It's basically a clone of Unix designed to run on desktops. Interestingly, Linux was originally intended to be a clone of Minix, but it's now far surpassed its predecessor in terms of usage and maturity.
All of these operating systems are based on Unix, to some extent, or are Unix-like operating systems. However, there are plenty of non Unix-based operating systems about, and many of these are also worth a look.
ReactOS is essentially an open-source clone of Microsoft Windows. It's designed to improve upon Windows while being 100% compatible with it. While this has yet to be achieved, it's certainly a promising OS.
Haiku is inspired by BeOS, which nearly became the OS on Macs (until Apple acquired NEXTSTEP and used that as the basis of Mac OS X instead). It's very lightweight and fast.
Finally, Syllable is a very fast OS that looks great.It's very fast and integrates the graphical interface with the operating system, so you don't have milions of window managers or toolkits to work around like you do with Linux. Check out this link for a YouTube video that demonstrates it.
Having read this, I hope I've piqued your interest! You may be worried about making changes to your operating system. Fair enough, it makes sense that you'd want to try something without risking your existing operating system. Fortunately, there are plenty of virtualisation tools that enable you to run it on top of your existing OS. An ideal one for beginners is VirtualBox, which will run on Linux, OS X or Windows. Others such as VMWare and QEMU are also available if you look. All of these allow you to run an operating system from an ISO image, so you don't even need to burn them to a CD. Just set up a virtual machine, select an ISO image to run, and you're ready to start! Plus, that way, even if something goes wrong, you can open a browser in the host OS to find the answer to the problem! It couldn't be easier!
So, what are you waiting for?