It's free. Very few things are free, and if they are, there is always some sort of catch. Not with Linux. Linux is completely free. No strings attached.
It's stable. If you run any version of Windows, you know that if you leave your computer on for a few days your computer will become unstable and will eventually crash. This does not happen in Linux. You can leave Linux running for days on end and it wont become unstable.
It allows for more configuration and customization then any other operating system. Because it is open source you can modify every aspect of the operating system. There are also many user created Linux modifications that you can obtain for free online.
It doesn't get viruses or spyware. Because of the design of Linux viruses can not exploit your computer in the same way that they can exploit a Windows computer. Because of this Linux does not need an antivirus or spyware removal tools.
It works on old and new computers alike. If your computer is brand new or ten years old, a Linux distribution will work on it. Not only will it work, but it will work well. If you have an old computer with limited resources you could use a light version of Linux that will cut back on eye candy in order to give you the best performance possible.
You support the Open Source community by using Linux. This in turn tells the Open Source programmers that you like the programs they are creating and that they should create more programs like it.
Many programs are free on Linux. Compilers, office applications, games, and browsers. You can save thousands and thousands of dollars in software by using Linux. And because most of these programs are Open Source they have no strings attached, they are really free.
It is faster then other operating systems because many programs can be custom compiled for your PC. Your compiler will use special tweaks and tricks to optimize programs for your processor architecture. This will result in your programs running faster on your computer.
It has a supportive user base. If you ever have a problem just go to a Linux support forum online and there will be dozens upon dozens people who will help you with any problem you might encounter.
Its logo. Windows has a boring multicolored window, Apple has an apple, Linux has Tux the penguin. Is there even any competition?