ubuntu is an actively developed Linux based operating system, with minor releases every six months, and major releases  scheduled well in advance. What makes this system different is that they have various versions: desktop, server, cloud, educational, and more, along with a click to install library of applications. There is also a commercial service related to Ubuntu (Canonical) where you can purchase full support. To make it simple to install in tandem on a Windows machine, they even have an installer that allows it to install beside Windows, through Windows.

http://www.ubuntu.com/