Yes. At my medical office and our medical research lab. Only slackware boxes and very glad with them.

Computer repair shop: two PCs running Slackware.

One acts as fileserver and hosts XP in VirtualBox with accounting software.

Other acts as router handling broadband connection and DHCP to Windows clients during repairs, etc., and remote X client to the server.

One PC (Slackware 14), set up as for Samba (PDC), NTP, and caching DNS. The server started life in 2000 as a Slackware 3.6 installation and has been moved from PC to PC over the years. Back then, there probably wasn't a "business Linux" and a "hobbyist Linux." You should give your Slackware some respect. If my support calls to Windows software vendors ended quickly once they learned that I used Linux on the server, it was because I was using Linux at all, not because I was using the wrong kind of Linux.

Deployed on more than 30 smaller networks (from 5 to 50 machines each). Mostly servers in *nix or mixed Windows AD networks. File servers (NFS, Samba), mail IMAP/SMTP (uwimap, sendmail), SQL databases (MySQL/PostgreSQL), backup servers, SSH, ... However newer contracts are based on FreeBSD, although existing Slackware installations do work well.

It is not much numberwise, but I admin a small civil engineering company. 25ppl approx 30pcs.
As the workhorse application there is AutoCAD the workstations nned to be windows based.
The files are usually very heavy though so the server is put to the test in this respect -all the work is done directly on the fileserver. But I have setup 3 of them with desktop level compoents that where laying around:

A firewall/gateway with ftp, ntp, dns,transparent proxy, vpn (anyone can work from home
a fileserver with samba,nfs,webdav, http for the intranet
a backup server with rsnapshots

It took me 2 months to set up the slack boxes (scripts especially). But I don't need to bother with these machines anymore. Oh wait, I did upgrade them to slackware 14 recently, so I guess I had to restart them...

Personally since I work in embedded development my workstation is a slackware, but alot of guides use debian based distros (but not debian!) to explain how to setup the environment, so regretfully I use ubuntu & mint for some of my work.

The car sharing system in my Kibbutz runs slackware.
The same system and other products of the same company are in use in many other Kibbutzim, probably running slack as well.

I'm a Systems engineer for an IaaS company. Everyone else in my dept. uses either Macbook Pros or high-end Dell Windows laptops. When I started there recently I tried the Macbook Pro they gave me for a month before giving up, giving it back (with great, great pleasure), and using my own Slackware ThinkPad instead. Everyone looked at me like I was deranged at first -- I think I'm the only Linux workstation user in the whole office -- but I can tell they are slowly getting more and more envious now they can see how superior it is in so many ways.

Currently I work as a ERP software developer for local ISP. The work is remote, so they gave me a ThinkPad T60 (old, but still great), which came with Windows installed. Since my employer doesn't mind what OS is used as long as the work gets done (we have people using Windows, Linux and Mac OS X), I quickly got rid of Windows and installed Slackware. Even though we have plenty of Linux servers (most of them Debian I guess, at least those that I work with directly), I think that I'm the only one using Slackware here. I'm not even sure if they're aware of it, they might be thinking I'm using Ubuntu or something.

I maintain servers at 2 architects offices, 1 civil engineering office, and a tax/bookkeeping office. I deployed Slackware.

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I am a small business of one, but all my machines are Slackware. I do quite a bit of modeling, simulations, and analyses and Slackware never lets me down. In fact, for number crunching work Slackware (or most any Linux) beats that other hobbyist OS.

One workstation in my office (14.0).
Four machines I use as a small cluster for off-loading calculations from the workstation (two 14.0 and two 13.37).
Two laptops when I travel to the customer sites (both 14.0). I dual boot the laptops with Win 7 for those occasions I need it at the customer's site.

I´m using 16 workstations in a cyber-internet cafe. My server is a SMS projet, or in other words also a Slackware!

I am running a small translation business which only has one Slackware desktop (Intel Atom-based). Some of my applications are web-based (notably accounting) which allows to log in from anywhere. I enjoy working with Slackware and have XFCE and KDE applications installed.

Yes, I'm running Slackware64-current as a freelance 3D animator.

And Mark is *so* jealous that he can't afford one for his own personal use.

I work for a fortune 100 company and I manage 7 Slackware servers in our local engineering group. I know there are alot of other Linux boxes at our company but I don't know all the distros.