Linux
A year ago, I noticed that all the programs I use are free, most even open source. The only one I pay for is Windows (being a software developer, I consider unauthorized copying one step up from cannibalism). I decided to migrate to Linux. Why? Generally, I prefer commandlines, and windows is very weak in that department. Cygwin is cool, really cool, but it seemed silly to keep windows as middle man. And cool as cygwin is, it is not the real thing.
Also, I like the idea of an operating system whose only purpose is to keep the user (me!) happy. I accept that Microsoft has numerous other priorities, which arguably come before users (making a profit, keeping partners happy, etc.), but I don't like the resulting Windows. There used to be no realistic alternatives. Now, at least for the technically minded, there very much is.
I decided to try out Linux on an old machine (a brick of a vaio from back when nobody was worried about Y2K). Big question: Which distro? A quick google revealed distrowatch.com, listing about a hundred times more distributions than I wanted to know about. Choice was easy in the end - Ubuntu is debian-based, has a huge installed base and is apparently gaining/maintaining market share. I could of course just use debian itself, and I might one day.
After one year, I can't remember needing to use my windows box for anything.
The single-most appreciated linux feature? Type apt-get update, then apt-get upgrade, and everything is up to date! Fantastic.
Linux is cool!