Red Hat Linux
Red Hat Linux is one of the "middle-aged" Linux distributions. It's not as old as Slackware, but certainly older than many other distributions. It was the first Linux distribution to use RPM as its packaging format, and over time has served as the starting point for several other distributions, such as Mandrake (originally no more than "Red Hat Linux with KDE"), and Yellow Dog Linux (basically "Red Hat Linux with PowerPC support").
See also