Recs.
Updated
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.
SpecsUpdate
Pros
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 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.
Cons
Con Almost no customizability
I should rephrase: it takes A TON of work to customize your UI. You're bound to run into hours upon hours of work because it's really unsupported and not very user-friendly at all. The ubuntu software manager is straightforward, though it has a pretty limited selection.