Skype is a P2P Voice Over IP program.

Skype is a free Voice Over IP program for Linux, Windows and
MacOS. It allows users to communicate with other Skype users
running any of the supported platforms, as well as receiving
and placing calls from and to normal land lines as well as
mobile phones.

This SlackBuild allows to build both 1.3 and 1.4 versions of
Skype.

Skype 1.3 needs QT3, so you'll have that installed (QT3 is
part of the slackware distribution).
Skype 1.4 needs libsigcxx (libsigc++), available on both 
SlackBuilds.org and linuxpackages.net).

Since Skype 1.4 is QT4-based, to keep dependencies at a
minimum the linked file is a static build, and is twice as
large as the 1.3 version, which is instead a dinamically
built QT3-based package.
The Skype 1.3 mirror is
http://ftp.surfnet.nl/pub/os/Linux/distr/pardusrepo/sources/skype-1.3.0.53-generic.tar.bz2
md5sum: e288e398e4a34a5760ff6321b253ed08

Both versions work fine with Slamd64.