On OS X, how can I see which song/stream/... is playing in iTunes?
Unfortunately, even the most recent release of iTunes doesn't have a very good method for this.

You could use the Mini-Player option, either from the Window menu, or toggle with option-command-M, but that is designed for album art - and won't show you the track info unless you mouse over it.

There are plenty of 3rd-party utilities, however.  I still use Bowtie for an inobstrusive floating desktop status, there are also others that can display your current track in the menu bar or via Growl, Mavericks' Notifications, etc.