CentOS (Community Enterprise Operating System) is a Linux distribution that is derived entirely from the sources of Red Hat Enterprise Linux (RHEL). CentOS exists to provide a free enterprise class, community-driven computing platform while maintaining very close compatibility with RHEL. Under a new governing board since 2014, it has joined with Red Hat but has remained independent from RHEL licensing and branding.
Please visit the CentOS website for more info and downloads.
Ubuntu is a complete Linux-based operating system, freely available with both community and professional support. The Ubuntu community is built on the ideas enshrined in the Ubuntu Manifesto: that software should be available free of charge, that software tools should be usable by people in their local language and despite any disabilities, and that people should have the freedom to customize and alter their software in whatever way they see fit.
You may download the .iso files for Ubuntu and acquire documentation, from our mirror here.