Well i can say that since i am a gamer is the main reason why i use Windows XP. No other OS is supported for pretty much all the games i play other than Windows. Linux is decent but to get it installed takes some knowledge. I havent tried too hard but since i cant game on it as easily as XP i just stick with XP.
As for the other apps well.....I use some M$ based apps only cause i am a Beta tester. Office 2007 is one of the betas i am testing ATM. I have tried Open Office and it is good. But for what i do Office is easier to accomidate.
Browsers well i use Opera overall. I do use IE7 Beta 2 as an alternative but Opera is my main browser. Other than that i use all sorts of apps from all sorts of companies/people. I use Thunderbird for email. XFire for chatting along with WLM. It all depends on what i am looking to do and what software is the easiest to accomplish the job with.