Bandana - $2-4, Bandanas are an essential part of my gear because they can be used for so many functions. I use mine as a scarf, hat, towel, pot holder, day pack, protective cover, mosquito net, etc. They are inexpensive too.
Platypus Collapsible Water Bottle - $15, Hard water bottles are essential, but these new collapsable water bottles are pretty sweet too. They are small and light. They can hold about a liter of water and as you drink the contents the form factor collapses.
Sawyer Squeeze Water Filter - $40, Previously I would buy bottled water when I traveled. Now I bring this small and light weight water filter and a bottle. It pretty much pays for itself after one week of traveling. The quality of the water is great and the world will be a cleaner place with fewer discarded plastic water bottles.
Leatherman CS4 Utility Tool - $50, It is hard to travel with a knife if you are flying and only travel with a carry on bag. But if you can check one of these handy tools through security it is so worth it to have available. I like this model because it has a can and wine opener.
LifeProof Mobile Phone Case - $90, Okay this is an item falls into the nice to have rather than need to have category. But if you are a media junky like me and love to travel with your iPhone this is perhaps the ultimate case. It is shockproof and waterproof and will charge your iPhone into a go anywhere device.
Round-The-World Ticket - $3,000-10,000, Obvious