Mac OS X Tiger, because its very easy to program.

At work:Windows Vista, Windows XP and Mac OS X 10.4.9.
At home
G5 Imac with OS X 10.4.9 and I quad boot my Dell with:
Ubuntu for internet browsing.
Windows XP for Pro Tools 7 not too mention it runs way faster on my PC than Vista.
Windows Vista: For DX 10 and people who think you have to have the newest Windows.
OS X 10.4.8 for Final Cut Studio 2 and Logic 7.1 and for My Mac freinds who think Macs are the best.

In order of Speed I have noticed that Ubuntu than XP; Vista and OS X run noticeably slower than those two.

Linux, cause it works ;)

sometimes windows2000NT, also very stable

XP at work, they don't know any better

anybody thinking of moving up to vista be VERY sure that your s/w and h/w is compatible 1st.

Amen to that.  Even Ubuntu, which is the most user friendly Linux I've tried using, has issues.  I have to jump through hoops to get my nvidia graphics card to work, and it still doesn't work in all circumstances.  Also, the wireless network adapter worked on the initial install, but after some time it just mysteriously stopped working.

Linux is great if you want to learn how OSes work and have lots of time to tweak it.  However, if you want something that works right away without having to go under the hood, you need either Mac or Windows.  With XP Microsoft made an excellent OS, and is also much more stable than Linux.  The stability issue may be a function of the software, though.  In general, Windows developers are more disciplined that Linux developers, which isn't saying much, but oh well. 

As for other options, I'll be running XP for at least another year or so, until Vista becomes more adopted.  There's still too much incompatibility out there.

OS X cuz Windows was pissing me off and now you couldn't pay me to buy a windows computer.......

Windows XP Pro on all my machines. I've never had any problems with it and despite what people say about MS, it's a good system. Before that it was Windows 2000 and then 98 SE. I really don't see why people run things other than XP. Everything works on it (well, maybe not now with Vista), it's easy to repair, and it is fairly intuitive.  Usually the only people that don't like it are the sheep that listen to the Mac commercials or the dorks that were told by someone they think knows what they're talking about that MS is the devil and open source is the ultimate best thing ever.

I've played with Vista and it looks pretty cool but there's nothing special about it yet. Give it a few years and it will be worth it; XP was bad when it first came out too.

I've played with Linux, and I know the Linux dorks are going to get at me for this, but it sucks. Yeah, it's faster because it doesn't have all the bells and whistles, but it doesn't just work. The programs it does have are generally bad knockoffs of MS products that don't have a lot of the good features and other things run in an emulator which makes it slower and buggy. I'm sorry, but I hate Linux.

I've played with Macs and have done a small amount of work on them, but I've never owned one. From what I have done on them, they're pretty and I'm sure once you learn where the shortcuts and where everything is they're fun to use, but something about them just doesn't feel right. I don't know if it's their keyboards are slightly different, the mouse has one button, or the fact I know I can't get in there and tinker with things, but I just never felt right using them.

Mac OS X and Kubuntu are my two primaries. Also on my 2 computers are 2 WinXP partitions, a Solaris partition, and an OpenBSD partition.

OS X. The only reason I got a Mac was for Final Cut Pro otherwise I would have bought a Windows system. However I found my Mac easier to use and more reliable than the PC's in my house so I'll probably carry on using them. I don't care about games on the PC so that isn't an issue.

Ubuntu, and typically install it on machines of others:

- rarely need driver disks
- can't get Windows virii

Windows 2000 proffesional, don't really care which one i'm using. Got it for free.