Portable USB battery. Comes in really handy. I love the small Anker I have.

Ziploc Space Bags. I'm definitely a minority on this one, but I don't backpack without these. They don't really wrinkle my clothes since I fold them well. They making packing really easy since they compress down to save space.

And not a physical thing, but downloading Wikitravel onto your Android or iOS device is a must.