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!