I work with a wide variety of client server deployments, and sometimes it isn’t obvious(via uname) what distribution and version a server is running. Here is a quick list of common files which contain that information:

Debian          /etc/debian_release, /etc/debian_version,
Fedora          /etc/fedora-release
Gentoo          /etc/gentoo-release
Mandrake        /etc/mandrake-release
Novell SUSE     /etc/SUSE-release
Red Hat         /etc/redhat-release, /etc/redhat_version
Slackware       /etc/slackware-release, /etc/slackware-version
Solaris/Sparc   /etc/release
Sun JDS         /etc/sun-release
Ubuntu          /etc/lsb-release
UnitedLinux     /etc/UnitedLinux-release
Yellow dog      /etc/yellowdog-release