I probably ought to say upfront, I'm not a complete linux/GPU obsessive, unlike some of my colleagues. I use different OSes on different environment (from WinXP, PocketPC, Symbian, Blackberry OS, Linux, etc). If I think a piece of software is worth the asking price, i'll pay it happily. I happily paid for Windows licences until recently, but, now-a-days, I believe the price/performance of Windows vs Linux isn't enough to make me buy Windows anymore.
I have a Windows XP licence at home, and used to boot into it to play games. And will likely do so again in future when I buy something which I have to run in Windows.
At present, however, I've not booted into it for months, as the only game I've been playing is World of Warcraft, which I can run happily under Linux.
I use a dual headed (Nvidia cards) Ubuntu box as my primary workstation in the office (I'm a technical project manager for a firm selling IT products in the Finance industry).
I have a 2nd box which runs XP, and Outlook 2003 (use synergy and the Linux box keyboard and mouse). I've tried using Evolution on the Linux box, but as it ties into the companies MS Exchange server via the Outlook Web Access system, it's inherently slower at retrieving email than the windows box, enough that I prefer to hang onto an XP box for my mail (admittedly, if I came to the company anew, and used Evolution from day 1, this probably wouldn't be an issue, but i'm used to the speed of Outlook on XP now so it's tough!)
My main applications on the linux box:
* Firefox - despite the numerous posts above, I rarely come accross a site I need to use, which operates poorly under firefox. All our internal web-based applications (and there are a ton of them) work perfectly well.
* OpenOffice 2 - I don't get the "it's a workaround" thing at all. Yes, it's a "clone" of MS Office from a few years ago, I realise that, and have no real problem with it. I can generate the same documents in it that I could in MS Office 2k3, plus I can publish everything in Adobe Acrobat from the button at the top, which is great when docs need to go out of the company). If I used Spreadsheets in a more hardcore way than I do, then perhaps I'd want Excel, it is undoubtedly the best spreadsheet around, but I'm not an accountant, so I don't. I find most non-accountant style spreadsheets people knock up in Excel would be better off as a database or other application.
* Emacs - fabuluous editor, never a moment when there isn't a session somewhere on my system, though admittedly takes some time to really get to grips with it, and to teach yourself to really understand its capabilities.
* Bash Shell - The power of the bash shell is something most people fail to realise, and is the main draw for me away from my XP box at work

* Wine - I run OpenWorkbench (think free MS Project, which happens to integrate marvellously well with CA Clarity, the online project management system use by my firm) under Wine, as I've not found a native linux port. It works fine, however, which is nice

* Eclipse - Only just started using it, not sure i'll be migrating to it from Emacs or not for my perl/phython/php/c/java development.
* Rhythum box / VLC media players (or whatever runs, I'm not too fussy so long as the tunes or video pops up

).
At home, I use largely the same software as above, plus:
* Google Picasa (hoping they release a PicasaWeb for Linux, as am not a massive Flickr fan, and would like to decomission my own Gallery server, etc).
* Wine running World of Warcraft
* The Gimp / XSane for importing piccies, sorting them out, etc.
Cheers,
Gav.