http://www.kernel.org – Linux Kernel Website
http://kernelnewbies.org – Tools and Resources for New Kernel Developers
Linux Distributions:
http://www.redhat.com – Major commercial Enterprise Linux Distribution
http://www.centos.org – Freely available Linux Distribution using Red Hat Enterprise Linux Source Packages
http://www.ubuntu.com – Linux Distribution provided by Canonical
https://www.debian.org/ – Debian
https://www.suse.com/ – Suse Linux – another commercial Enterprise Linux Distribution
https://www.archlinux.org/ – Arch Linux