REI Co-op Expandable Packing Cube Set, $44.95
If you're traveling for a prolonged amount of time, your suitcase (or backpack) is more than just a place to hold your things; it's pretty much your life condensed into a reasonable-enough-to-carry size. Make the most of what space you do have, and bring some routine to an otherwise unpredictable time with packing cubes. I brought these with me specifically because they're waterproof and expandable. You don't have to worry about any toiletry spills or leaks ruining your clothes, and you can seriously maximize your space with the expandable zipper.
Packing cubes also make finding all of your belongings much easier. I always knew where I could find my shirts, shorts, socks, etc., because I packed my cubes accordingly. If you're frequently traveling to new destinations, having this organizational system will make your life a lot easier. Instead of wasting time shuffling through your bag to get dressed when you touch down in a new city, just quickly go into your packing cubes, get what you need, and you're all set to explore an exciting new spot.