As much as enjoy streaming music using AirPlay to speakers around the house, fact of the matter is iTunes has become a clunky, bloated piece of software that's both functions as a storefront and a media player. Despite having plenty of memory to run iTunes and more, we wanted to see what else was out there for those who didn't.
The biggest draw for alternative players is upsampling and access to audio filters built into Mac OSX (yes, audiophiles, this article is for you). Things such as a 31-band equalizer and locked sample rate to the track being played - advanced options like these never see the light of day in iTunes. A disclaimer: we've only read about most of these programs, so proceed at your own discretion!
Amarra by Sonic Studio: This commercial software basically acts as an attachment to iTunes and utilizes some fancy iZotope resampling software for the ultimate playback experience.
audirvana (Open Source): This free player looks like a piece of hi-fi gear and offers upsampling for no cost at all.
Clementine Player: Also free and open source, this multi-platform player is more of a music organizer and is pretty snappy.
Decibel by Steven F Booth: Another free one which has a history of being called "AyreWave" at one point in time, acts as a basic media player with one-click importing of files in Finder or iTunes.
Ecoute by PixiApps: Probably one of the more attractive than functional apps, Ecoute integrates with iTunes and provides a slick, themable window interface to rate songs, share listening habits on Twitter, Facebook, or LastFM. Unfortunately, this pretty add-on will require some cash.
Fidelia by Audiophile Engineering: This commercial product also mimics from high-end hi-fi gear, has the ability to play music directly from iTunes playlists without ever needing to fire up iTunes. It comes with an array of resampling methods and has remote control software for the iPhone/iPod Touch.
Play by Steven F Booth: This free package basically turns iTunes on its head and shakes everything out, leaving only a bare minimum music player. Although, you do have access to a 31-band equalizer if you wanted to.
Pure Music by Channel D: Like Amarra, this one runs alongside iTunes and resamples stuff on the fly, but I personally think Amarra looks way better.
Songbird: Made from previous Firefox, Netscape and Winamp developers, it's pretty much the "anti-iTunes" of this roundup, even though a lot of the buttons look exactly like iTunes' playback interface.
Vox by Alessio Nonni: This free player is much like Play (dumbed down music player) with a minimal footprint.
That's it for now. If you've got more recommendations for your fellow readers, we'd love to hear 'em!

Ercol Bar Stool
No love for Banshee? Available from banshee.fm
Banshee is written in C# (using the Mono platform on Linux and OS X), using GNOME technologies (Gtk#, GStreamer, etc), and SQLite. It is free/open source software, released under the MIT/X11 license.
I use it at work every day on my macbook. It also interfaces with just about every portable device.
Cog is a decent alternative, too.
Especially the nightly build I have been testing...
iTunes is my favourite piece of software. I shop, I listen, I watch, I organise my phone... I do nearly everything. It's like my media dashboard.
Won't be switching for anything. ^_^
I certainly agree about iTunes bloat and marketing focus. But what I don't understand is why write an article beginning with "A disclaimer: we've only read about most of these programs, so proceed at your own discretion!"?? If you haven't tested them you're just rephrasing what others have written and how useful is that? Really, why did you bother? Space filler?
There is also a player for Mac OS X, it's called G-Ear, and is constantly evolving! Available at www.treasurebox.hu
In all players I have tried so far, I was always missing something. Either the simplicity, the openness (Open Source), the support (flac or ogg) or a decent PartyShuffle mode.
Thus, I started my own project which provides everything I wanted. It's Open Source / free, so you might be interested in it.
MusicPlayer for Mac (Linux/Windows works in a limited way at the moment)