OS X alternatives to iTunes for playing FLAC

There's Clementine, a cross-platform fork of Amarok 1.4. I'm not sure how well (if at all) it works with iTunes playlists, etc. but it does support FLAC and has (in my opinion) a great library management feature set.

Enqueue does everything you want and the nice thing is that you can assign any folder or folders to it and it will watch them.

Extensive File Format Support
Enqueue supports tagging and playback for many formats including mp3, mp4, aac, ogg, flac, wav, aiff, musepack and more.

I've only used the beta which wasn't completely stable, but it is out of beta now.