Slackware Linux is still going strong. The Slackware Linux Project released Slackware 13.1 on May 24th.This is just a little by shy of the 17th anniversary of Slackware 1.0 and not quite a year after Slackware 13.0. Delve back into yesteryear with me; you're in for a treat.
Using Slackware is a little bit like entering a time warp, back to a simpler time when Linux was not being backed by billion-dollar companies.
Other distros offer streamlined GUI installers that leave few decisions to the end user. Slackware, on the other hand, provides a very basic installer with text menus that leave most of the decisions up to the user. In fact, when you boot from the Slackware CD or DVD, you'll be told to partition the disk yourself with fdisk or cfdisk and then run setup after partitioning.
The Slack Experience
Partitioning the hard disk is the most challenging step of installing Slackware, which is to say that it's not really that challenging. But it might be a bit off-putting for users who haven't done it before. Once the partitioning is finished, the setup routine is very straightforward. You'll need to set up networking, passwords, and your software selection. Unless you have a very small hard disk, it's probably best to just select the full install of Slackware. A full install takes up about 5.5GB of drive space, which is not a great deal of space on today's systems.
After the install and reboot, users are presented with a bash prompt after login. During the install, Slackware asks which X window manager or desktop environment they'd like to use. But it defaults to a multi-user runlevel without a display manager. If you want to change this, you need to edit /etc/inittab and set runlevel 4 as the default. Otherwise you can fire up your desktop using the venerable startx command.
That pretty much defines Slackware. Want to modify system behavior? You're going to have to roll up your sleeves and dive in. Want to install software? Get used to using pkgtool and other command-line tools. You can also use pkgtool to re-run some of the initial setup scripts, like if you want to change the network settings. One thing that has changed recently is the inclusion of slackpkg, which is similar to Debian's APT. It's not quite as full-featured and dependency resolution (making sure you have all the necessary software to run a package) is not built-in, but it is handy for installing software off the network.
The default selection of software for Slackware, which in my case is the whole kitchen sink, includes several window managers and desktop environments. You'll get KDE, Xfce, Windowmaker, and a few others. The most recent Firefox is included, and KOffice, the GIMP, and quite a few FOSS staples. But you'll need to look elsewhere for GNOME, OpenOffice.org, AbiWord, Gnumeric, and many other popular packages.
Even if you install the kitchen sink, a few things are held back in the extra directory on the CD. This includes stuff like official Java, an MPlayer browser plugin, and a script to create a Slackware package out of the current Flash plugin for installation.
The nice thing about Slackware is that you get to see upstream software pretty much as it was shipped by the projects. Most of the major distributions spend time modifying, customizing, and polishing the upstream software to tailor it to the audience they're trying to serve. This means adding and removing features, ensuring that packages are well-integrated, and branding the software. So the KDE you use with one distro isn't going to be quite the same from distro to distro.
Slackware, on the other hand, takes a very light touch with upstream software. There's very little, if any, branding to be found. Software is mostly presented just as the upstream projects shipped it.
It's All About Audience
You've probably surmised that Slackware isn't for everyone as a full-time OS. It's not. Slackware is for users who like a basic, solid Linux distribution that makes very few assumptions about how the system is going to be used. That approach means that Slackware is perfect for a small percentage of Linux users.
Still, I think Slackware has a certain style that should be appreciated. Criticizing Slackware for lack of modernity would be like criticizing a well-maintained 1957 Chevy for not having power windows or satellite radio. You don't run Slackware to escape from the complexity or configurability of Linux; you run Slackware to embrace those things.
Users turn to Slackware for a Linux distribution that doesn't get in the way. Package up the system software and make it relatively easy to shove on to a computer. Then get out of the way. And that's what Slackware does.
If you've never tried Slackware before, you really should. Find an older machine, or whip it into VMware or Virtualbox and give it a go. Take the time to set up your system just as you like it, and you'll learn a thing or three about Linux in the process. It's a great distro to get under the hood and dig in. At the very least, you'll have an idea how Linux has evolved over the years. Aside from newer versions of everything, Slackware has changed very little. That's not a bad thing.