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The most popular Linux distribution, Ubuntu offers different OS versions for various devices like desktops, servers, tablets and phones. The desktop version, a fork of Debian, is very stable, particularly due to the number of active users, and provides a lot of features out of the box, so you can start working without having to install hundreds of drivers, tools and software first. For a lot of people, Ubuntu is the most competitive free OS to Windows and Mac.
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Pros
Pro Just works out of the box
Lots of support for hardware, lots of pre-installed software, and a smooth install process means less time downloading drivers, less time digging through configuration files, and less time deciding on software to use just to get up and running. It also means less time digging through forums looking for support.
Pro Lots of support
As the most popular Linux distribution, there's a wide range of sources for support online if you ever need help, including the Ubuntu Wiki, Ubuntu Forums and the Ask Ubuntu Stack Exchange site.
Pro Dedicated software center
The Ubuntu Software Center offers a GUI interface for installing new apps which is extremely easy and welcoming for beginners to Linux. But it might not be used by more advanced users since the method of installing through the terminal is much faster and easier after one is used to it.
Pro Use it on almost any device
Not just for laptops - Ubuntu was designed with tablets and touchscreen devices in mind, and with phone support on the way. Ubuntu also has Long Term Support releases, as well as a version oriented toward servers, so you can use the same OS at work or on mobile as you do on your desktop.
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Flagged Pros + Cons
Pro Great long term support release schedule (2 years)
This allows for users to always have a new supported release available without long unkown wait times in between.