Debian /etc/debian_release, /etc/debian_version,
Novell SUSE /etc/SUSE-release
Red Hat /etc/redhat-release, /etc/redhat_version
Slackware /etc/slackware-release, /etc/slackware-version
Sun JDS /etc/sun-release
Yellow dog /etc/yellowdog-release
You can find more information on this problem as well as a shell script to assist with OS detection here. An even larger master list can be found here.