http://www.foobar2000.org/ is a fantastic piece of software. Extremely efficient/powerful/customizable (and closed-source). I sorely miss it under Linux, Rhythmbox/Quodlibet/Amarok/Clementine don't even come close.

What powerful features does one need in music player? Or, what features do you miss in OSS players?

WMP actually isn't that bad these days, though all I use it for is ripping CDs (yep, those shiny plastic things for you young'uns) since it's great at that. For playback in Windows, I've stuck with Winamp through the years as I haven't found anything that works the way it does without a bunch of features I don't need. I tried Foobar2000 and couldn't get into it, and MediaMonkey just seemed like overkill.

By the way, a great site for finding apps in specific genres is alternativeto.net. Here's a list of alternatives to iTunes:

http://alternativeto.net/software/itunes/?platform=windows

I'll never understand this community...downvoted because I use WMP (oh the horror), or downvoted because I was trying to be helpful?

I moved to Linux (Ubuntu) and migrated a HUGE iTunes library along with it. I now use Clementine as my music management / player. I believe there's a way to import an iTunes library, but the iTunes metadata is all in one big XML file. So I wrote a script that loads all my metadata into Clementine's sqlite database.

I can't tell you in words how awesome this is.

http://www.jriver.com/

But I have used it for 10 years and it's the only reason I still have Windows machines in my house. It's that good.

Winamp is still my preference.