Quick-Dry Towel
Flicker
Most hostels offer towels these days, but there's a good chance you'll have to pay to rent it. Rather than paying and risking an immodest scurry down the hall in a too-small hostel towel, carry your own quick-drying towel. They absorb tons of water, and as an added bonus, you'll have a beach towel or a blanket to lay on in a park if you need it.
Small Headlamp
Wikimedia Commons
There's nothing worse than trying to hunt through your entire bag in the pitch-dark to find your toothbrush (and nobody wants to be that guy who turns on the overhead lights at 2am and wakes the whole room). Instead, carry a small headlamp and always keep it in the same pocket of your bag, so you can find it. In Southeast Asia and parts of Africa and South America, power outages are a common occurrence, so it's also nice if you're headed there to have some insurance in the form of a light. Pro tip: Check out the kids' head lamps on Amazon; they're cheaper and smaller than "adult" ones, though no weaker, and they come in sweet patterns like dinosaurs and flowers.
Clothesline
Pixabay
Every now and then, you'll find yourself needing to wash a few things when you can't get to a laundromat or none of the hostel's machines are available. Rather than decorating your entire dorm with your unmentionables, hang them from your braided elastic clothesline. This type of line can be strung from just about any two small poles, making the long end of your bunk bed a great candidate. The braided cords make it so that you don't need clothespins to keep things secure.
Luggage Lock
Flickr
A double-ended bag lock is the ultimate multi-purpose device, and the only lock you need. You can use one end to lock your bag's zippers closed and the other to attach your bag to a bar (like those on your bunk bed, a luggage rack on a train, or a lounge chair at the pool or beach), or you can use the lock to attach your bag and a friend's bag together -- making it just about impossible for a would-be thief to pick them up and carry them off. And of course, you can also use this type of lock on hostel lockers.
Flip Flops
Wikipedia
Rubber flip flops are one of the most quintessential hostel must-haves. They'll protect your feet from the germs on shower and bathroom floors and are easy to slip on when you just need to drag yourself down to breakfast before it ends.
Hand Sanitizer
Flickr
You never realize how often you need to wash your hands...until the bathroom is all the way down the hall. It's also nice to have hand sanitizer before digging into street food or a snack on a bus or plane. Spray sanitizer tends to last longer than the gel kind and is less likely to pop open and leak in your bag.
What Not to Pack
Sheets
Wikimedia Commons
Almost every hostel we've encountered offers sheets these days -- often for free. If you know you will (or think you might) be staying somewhere that doesn't have sheets, at least opt for a sleeping bag liner instead of a full sheet set. Liners are so much smaller and lighter, making them easier to pack and wash, too.
Pillow
Flickr
This may be a divisive one, because we know plenty of travelers who insist on carrying a travel or full-size pillow, but hear us out. On long-haul flights and overnight buses and trains, pillows are provided. If you're just traveling for a few hours, a hoodie can work just as well as a pillow and is way more useful, and if you can't sleep on the occasional flat, lumpy pillow, you just may not be cut out for backpacking. Do we even need to mention the grime and filth you're dragging your pillow through when you carry it with you through the streets and public transit of the world?
Money Belt
Flickr
Like a travel pillow, a money belt is a "must-have" for many, but we think you'd be better off skipping it. Frankly, the least safe place for your passport and valuables is on your person -- yes, even in a money belt. They're much safer locked (did we mention locked) in your bag or in a locker at the hostel. If you're still wary, invest in a travel safe, which will also hold your laptop, tablet, e-reader, and other larger valuables. But seriously, that money belt is not as inconspicuous as you imagine, and you definitely don't want to be worried about not losing your passport and hundreds of euros when you're out on a pub crawl.
So, are you packed and ready or did you just hop on Amazon to see how quickly you could get a headlamp shipped?
Kelly Bryant