I must confess that I am biased. Horribly, irreversibly biased. I love the Free/Open Source software world because it provides the ultimate in pro-customer values. This is a huge world that encompasses GNU and the Free Software Foundation, the Open Source Initiative, thousands of smaller independent projects, corporate contributions, and the Linux kernel. For convenience's sake I'll lump all of this incredible richness under "Linux."
When you build your infrastructure around the Linux operating system, you get the ultimate in flexibility. Linux servers don't care if you want them to serve Linux, Unix, Windows or Mac clients. Linux desktops don't care if they have to rely on an Active Directory domain controller for authentication and access to resources, or if the get to play in an all-Linux environment, or have to tug a forelock to a mighty Unix server.
Linux itself doesn't mind being scaled up to power a mainframe, or a cluster, or a multi-processor server, or squeezed into a PDA or cell phone.
Linux doesn't try to lock you into a particular Web browser, office suite, desktop environment, groupware, mail server, graphics editor, sound editor, CD/DVD writer, or mail client. It doesn't even try to lock you into using only Free/Open Source software – if you wish to use closed-source proprietary software, or even software written for another platform, Linux tries to accommodate you.
Linux doesn't infest your system with all sorts of commercial-ware that phones home who-knows-what.
You can have both thin and thick clients on your LAN, and you won't need a math PhD to calculate the licenses.
You can run virtual Linux servers, and you have several to choose from.
You can run a complete Linux distribution from a CD without installing it. Bootable Linux CDs make great rescue disks for both Linux and Windows. There are hundreds of different bootable Linux distributions on CD, DVD, diskette, and removable flash drives, and if none of them suit your needs you can easily roll-your-own.
If you publish descriptions of security flaws, or break encryption schemes, you won't be threatened with lawsuits or criminal prosecution.
If you share ideas, knowledge and improvements, you will be greeted with approval and more sharing, rather than threats of punishment.
Linux doesn't care what region your DVDs are encoded for.
Linux doesn't try to trap your data in closed, secret, incompatible file formats.
Linux does not try to extract as much personal information from you as it can get its sticky little fingers into; rather, you are encouraged to protect your own information with encryption and other safeguards that you control.
Linux doesn't try to control and commercialize every last minute aspect of your computing infrastructure.
Free/Open Source software comes in various permutations of free-of-cost and free-as-in-freedom. Some commercial distributions let you buy it once, then install it on several machines. Some are more restrictive, and the free-of-cost distributions encourage you to copy and re-distribute them as much as you want. Whatever you prefer, you'll probably find it.
If you wish to study and modify the source code of some of the best programs in the world, you won't break any (increasingly ridiculous) laws. If you want to build a test lab and try out all kinds of different network configurations, servers, and applications, you will not run afoul of the license police.
If you want to understand the inner workings of software development, follow mailing lists and online forums. You'll be hobnobbing with coders at all levels, system administrators, and other users, all without corporate muzzles.
If you wish to be a part of the incredible creative ferment of the Free/Open Source software world, you'll find a welcome somewhere. (Note: manners do count. Only manner-less nitwits believe courtesy is disposable.)
Linux vendors are not hostile to their own customers; you will not be bullied, bribed, extorted, or threatened. (Sorry about the lack of bribes part.) SCO doesn't count; they're not a Linux vendor, they are litigation specialists.
I'm not claiming that the F/OSS world is Utopia. After all, it is populated by humans. But it is a very wonderful place, and for people who want to control their own data and computing infrastructure, it the very best place to be.