What Linux release am I running?

For every Linux distribution/distro there seems to be a different way to work out which release version/name/codename you're running. Here's a the ways I've found, on the command line, to find out:

Ubuntu

lsb_release -a
cat /etc/lsb-release
cat /etc/debian_version

Note: For Ubuntu /etc/debian_version seems to contain the Debian base from which this version of Ubuntu originates.

Debian

cat /etc/debian_version
cat /etc/lsb-release
lsb_release -a

Note: The latter two are not always available on pure Debian systems.

Redhat/CentOS/Fedora

cat /etc/redhat-release

Gentoo

cat /etc/gentoo-release