The oldest distribution still out there and getting better all the time.
Slackware is the second Linux distribution to be available to the masses. (the first being Software Landing Systems - SLS, which Slackware derived from.)
There are no segmented distributions (i.e.: "Professional", "Server", etc.) as everything is included for just about any install scenerio, whether workstation, server, firewall, etc.
You do have to be intimate with your hardware and you should know a little something about *nix in general. This is not a glitzy "Look at the fancy GUI while I throw everything on your system and configure it the way we think you should run your computer" installation.
(My two cents, your milage may vary. )