I happen to be using the oldest surviving Linux distribution, Slackware, on my servers. I have been using Slackware for a very long time; in fact, the only other distribution I ever used was the first Linux distribution, SLS (Softlanding Linux System), which was ultimately succeeded by Slackware.
Now I realize that while Slackware is perfect if you actually know what you are doing, it is not the easiest distribution to use. It lacks many of the system management, package installation and dependency resolution tools that users of more recent distributions take for granted.
This is why I was very surprised when I read this morning in PCWorld that in a recent survey conducted by LinuxQuestions.org, Slackware was found to be the most popular desktop Linux distribution. I may have expected to see Slackware fare well on servers, but the desktop? Mind you, I am very pleased to see that Slackware is doing well, even though it appears to have been a somewhat informal survey.
As to servers, Slackware came in as a close second, narrowly beaten by Debian. Even a second place finish is impressive for this venerable distribution.
I just hope that Slackware is here to stay for a long time to come. I would loathe to switch distributions after all these years.