Slackware only. But I have had Zenwalk and Vector installed in the past, and recently tried Salix in VirtualBox.

I use a stock Slackware system, but if I need some software that's already being provided by one of the derivatives (usually Salix or Zenwalk) I'll grab the slackbuild from their repos and compile it myself. That way I get the best of both worlds: a stable, well-supported, constantly-updated distribution while not missing out on software that I want/need.

Slackware only at the moment (and antiX). I' ve tried Zenwalk for some time, but at the end it was not meant to be.

Slackware 13.37 on four boxen, one box with Slackware-current. I don't use any derivatives.

Multilib Slackware64-current on my main box and Slackware-current on 2 older machines (P4-3GHz and PIII-550MHz)

slackware only

Slackware only.

Slackware only here, too

Slackware only, but I sometimes take a package from the SalixOS repositories if I cant get it from slackbuilds.org.

samac

One of my computers uses Salix, the rest use slackware.

I use slackware and a number of live systems based
on slackware: slax, RIPlinux, pmagic, ...

Slackware only here too.

I use Slackware and Slackware64. It has been a few years since I last booted from a Slax CD. I did take a script from VectorLinux last year. But a derivative installed and in use? Que?
The lack of a Slackware only option is a serious omission.

PS - This is starting to look like a subscriber list.

Pure Slack.

*Embarrassed face* Salix 13.1 for some months, now Salix 13.37 xfce.
I personally think it's not so far when Slack will be my main desktop OS. :) *touchwood