4) For portable devices, when I switched from Windows XP to a Mac Mini for managing and syncing my music was by far the best thing Apple has done for me.
3) The Airport Express added an additional flexible to have music running throughout the house for a very reasonable cost. Whole House Audio for less than $150 per room.
2) The "remote" app to control the ATV, Airport Express. Also mPod for the MPD daemons is rather good.
1) I've used USB DACs and the optical output, with the Mac Mini running Linux and Mac OSX, but the biggest gain I've had was when I switch to the Bryston BDP-1/BDA-1 combo. The Mac Mini still plays a huge role by performing the Hi-Rez downloads, ripping CDs, primary storage, backup and even automated the syncing of new songs to the BDP. The dead head mac mini now sits neatly atop a mac pro in the corner of my desk.