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.
Snow Lizard Waterproof Solar Charger Mobile Case - $150, 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, waterproof, contains an extra batter and will charge your iPhone with a built in solar cell. No worries about lacking battery power in the wilderness.
Round-The-World Ticket - $3,000-10,000, Obvious