It's not necessarily going to replace iTunes, but Vox is a very tiny, very capable, very free music player and converter that is far more powerful than it looks.
You play music in Vox by simply opening up an audio file (in practically any format). While you can't make a custom playlist, Vox will now have quick access to any music in the same folder as the song you opened, so you can create playlists simply by throwing some music together in the Finder. Beyond simple playback, Vox also has a powerful 10-band equalizer and several effects (reverb, time stretch, pitch shift, etc) that can be applied to your audio. At any point you can export any song (or the entire contents of the current folder) to AAC, AIFF, and a variety of other formats. The export can simply transcode the audio file, but it can also apply your EQ and effects settings to the new file as well.
While it's not necessarily a solution for managing enormous music libraries, Vox is a great little music player for playing music you've got just about anywhere. If you take you don't like managing your music at work and just take a hard drive of MP3s with you, Vox is a really great way to play them.