3rd party repositories and unofficial (!) package management tools, I use:
For original sources, for which no SlackBuild script and no Slackware package is available, I frequently use src2pkg
My favourite repos are, however, Alien Bob's
and Robby Workman's
I have also used slapt-get
in the past, mainly for packages from http://linuxpackages.net
, but prefer Slacky.eu/slackyd now.
An overview of tools for package creation, conversion and management seems to be this Slackware related blog
Be careful with tools, you are not familiar with, though, and with software from less popular repos. Packages from SlackBuils.org, slacky.eu, Alien Bob and Robby Workman, plus a few others can generally be trusted. But in the end, *you* are responsible for your system, and this responsibility cannot be delegated!