If you buy Slackware directly from the web site, you get a fourth CD called "live." It's basically a full Linux on CD for demos and what they call "the ultimate rescue disk." Other than that, there are only 3 CDS and you really only need the install one and there are a few interesting things on contrib. Oh, Slackware 8.1 is in beta now and it looks like it's ballooned to 2 CDS. They are adding some stuff like Logrotate and it will come with kernel 2.4.18 unless a newer one comes out before it does.
