Okay I have tested many different OS's including every Windows OS (starting from 95 lol). I have tested many many (over 50) different distributions of linux and well one unix. I have switched back and forth from windows to linux and could not decide which i like better...until recently. I am currently using Ubuntu 9.10 Karmic Koala and find it very appealing and I wish to share my experiences with all you who have no clue about linux or just haven't found a distro that suits your needs yet.
History
Linux was first developed as a substrate from Unix in 1991. The goal was to create an OS that used all free software under the GNU General Public License and to make a terminal based OS that was very useful in networking and developing situations.
Today it has forked into many different pathways from visually enhanced to very complex. There are many different distro's, the main one that many are derived from is the Debian build. Though they all use the same kernel, there are many different variations that come pre-configured to work for certain situations. Ubuntu has become very popular in its stability for server editions and graphical front-end for desktop editions. It was derived from Debian.
Desktop Edition
Today there are many features that make this a user friendly OS. One of the key features that linux distro's offer that is not included in any Windows Installations; is the option to mount the kernel to RAM (Live CD) and see what the distro has to offer before installing it. For years there has been a compatibility gap between Windows and Linux due to the fact that most programs are designed to be used with either Windows or Mac. For those technologically advanced that preffered the linux kernel; they would spend days re-adapting windows coded applications to be compatible with the linux kernel. This proved to be very problematic and provided no solution to the everyday user. Finally Wine, a windows program emulator, was developed and works with many windows applications that are C++ and C# based. I myself use Wine to run World of Warcraft with increased framerates in linux. Enough with the history lesson, lets list some advantages.
-Synaptic Package Manager/Aptitude; many different GPL programs designed specifically for the linux kernel are featured using this and can be easily downloaded and automatically installed (very user friendly)
-Support for nearly every device; Unlike Windows, Linux features many generic drivers that are automatically incorporated to work with your devices. There is even a restricted drivers feature that allows you to download specific drivers for certain hardware such as display drivers.
-Ohh so pretty; The Linux X-Server (screen output server) just seems to give better graphics than Windows. There are various window managers out there that give cool and zazzy window and desktop effects. Even the modern day Windows 7 does not seem to compare to Linux graphics.
-Memory, CPU, and GPU Conservation; Linux by nature uses less processing power and less RAM than any Windows OS's. The kernel is built on this feature and has a much smaller front-end than Windows. Your GPU also uses less power because of the powerful yet conservationalist nature of linux. Call it an all over power and frustration saver.
-Personalization; The Linux kernel offers many different window managers that can offer the right look for any audience. Some of these include XFCE, KDE, Fluxbox, GNOME (My favorite), and many others that I myself have not tested. Using the GNOME window manager you can personalize panels (AKA taskbar) and have one at the bottom, customized like Windows; or one at the top for menu and notification functions and one at the bottom showing your open windows (The GNOME standard).
I'm sure there are more but I am tired of writting about it (carpeltunnel?). Though this guide may make Linux seem really user-friendly please note that : If you have never used it before, do not charge into it or you will do serious damage to your computer. Linux is not for the easily frustrated. Make sure you google some guides and references to Linux to familiarize yourself with it before attempting to install. I am not liable if you decide to type something obstructive in terminal not knowing what it does...
Server Edition
I currently use Ubuntu 8.10 Server Edition for my ArcEmu core WoW server. It is very fast and efficient to the knowledgeable user. ArcEmu offers an extensive guide on their wiki to installing it which I find myself refering to alot even though I know how to use the terminal side of Linux. DO NOT, I repeat DO NOT attempt to use a Server Edition of Linux without knowing extensively how to use terminal and having another computer or phone to lookup some things you just cannot figure out yourself.
Other Uses
Since the Linux kernel is small enough to be completely loaded into RAM of even older machines, there are many tools that use the Linux Kernel. These include tool such as : Windows Password Crackers/Recovery and Backup Recovery Disc (Such as the one used for Macrium Reflect). There is even a distro of Linux designed specifically for older computers called DSL (D*mn Small Linux) that offers a many tools, games, and complete X-Server in a 50 mb image. I'm sure many other uses (especially hacking) exists that I am not familiar with. Just use your friend google XD.
Credits:
-Me for typing everything
-Wikipedia for the 1991 date; the only thing I didn't pull out of my head
Note: I did not double check this because it soo long, feel free to correct me. A few capitalization errors I realize...
Edit: Ah yes I forgot a big plus for me was that the new Ubuntu has an automatic configuration tool to set up mobile phones for dial-up use. :)