Eye shades and ear plugs for flights where you want/need to sleep. Sleep aid helps (couple beers pre-flight and a couple Nyquil tends to do the trick for me).

Agree with the adaptors! ^ Even if heading to a place with like power plugs, you never know when plans change and where you'll end up.

Excedrin Migraine -- cures the headaches from work and the extra glasses of wine after work.

Bottled water -- I always pick up at least a liter after security. The dry atmosphere in the planes plus the aforementioned pre-flight beers do not mix well with proper hydration.

And the most important accessory is your photo ID! You can buy nearly everything else where you are headed, most of the time, but will get nowhere without that.