What OS do you run?
In the office I have both debian and Windows XP. My office network (20 people) has everything from AIX to SuSE to Windows NT, but most desktops are 2000/XP and most servers are 2000/2003. I also have a couple of webservers, also running debian.
At home, I have a debian machine, a dual-boot debian/XP PC, an always-XP machine, an iLamp with OS X and a PowerBook soon to be running debian (OS X is just too slow on it).
Why do you run it?
For a few reasons:
As a server and a desktop, I can update and maintain my linux machines consistently. They are easy to update.
As an internal server, linux is a workhorse, and runs a lot of the development and workgroup tools that we need better than Windows ever could.
As a web server, I know I can trust the platform, and I can firewall it pretty easily if need be. Again, updates are an issue, and low-bandwidth remote access (over SSH in this case).
As a firewall/router, some of the same issues apply. I can trust exactly what I've installed on the box, I have full control over the protocols and methodology, and I can do a lot more with it than an out-of-the-box router.
Why debian? Well, updates are easy to apply, I know how to use and abuse everything, and I have an extensive understanding of the platform. Easier to run what you know, though I have experience with other platforms.
I run Windows XP because, of the Windows family, it's the best they've got and has resolved lots of minor issues that may never be resolved in Windows 2000. It's a lot more "plug and play", which removes some of the frustration in not being able to poke and prod it the way I want.
The majority of my infrastructure in the office is still Windows because it works with everything my users want on their desktops and everything they need to use. I don't have the time to deal with workarounds, emulated software, and other issues that would come up with switching to all linux on the desktop. On the servers, Windows just plays best with Windows, and I don't have a lot of time to deal with doing it any other way... just yet
I have a couple of Macs because I really appreciate the platform, especially OS X.
When did you start using it?
I started using Linux sometime around 1997 or so, around the same time I really started using Windows. Before that it was only casual occasional use of Windows on library PCs and in a couple of classes, and a ton of Mac exposure (where I did all of my homework and played some games).
Outlook 2003 on Windows, Evolution on Linux.
Audio and video players?
Hmm, I don't do a lot of AV stuff day to day. I do use XMMS, iTunes, and WMP, when the need arises.
Office progs (editors, etc.)?
Office 2003 in Windows, OO.o on Linux, Office X on my Mac.