External hard drive for Time Machine!
I have a trackpad, but totally never use it. I love the one on my MBA though, so not too sure why the desktop version never clicked.
I also have the Apple battery charger, but it's probably a tad overpriced honestly.
Thunderbolt to HDMI adapter (either Monoprice or Amazon).
And depending on your uses, an Apple TV is awesome. My media library is on my iMac, so they work together flawlessly!
I also just got a portable blu-ray drive for backing up my blu-ray collection. Definitely helpful.