After over half a year traveling with one backpack full of necessities (and some deadweight), I figured I should advise others on what some of the essentials are when planning a long-term trip while carrying (or wheeling) your belongings with you.
There were definitely things I was glad I brought, things I threw away along the trip, things I wished I had, and things I kept, hoping I'd need them along the trip.
TOP 20 Essential Items
1. Coghlans Pegless Bungee Clothesline
If you plan on washing your clothes on your own as you travel for long periods of time because you want to save money or because washing facilities are unavailable, this is a very functional, effective, and adaptable clothesline to dry your clothing. Using 2 yellow elastic bungee cords wrapped around each other, clothes can easily hang in between the cords without the need of clips. Staying in hotels and/or guesthouses, it was easy to find places to attach the clothesline, whether it be on doorknobs, chairs, or towel racks.
2. Elgin Travel Alarm Clock
The alarm clock is small enough and lightweight to carry throughout your trip, and is reliable for the days you need to wake up to catch a plane or bus ride. The clock folds to a compact size that prevents the user from accidentally clicking the buttons. When the clock is unfolded, the case serves as a stand. The blue glow of the screen is easy on the eyes, especially when your eyes are accustomed to the darkness. Uses 1 AAA battery, which I did not need to change in 7.5 months. The alarm was loud enough to wake me from sleep without scaring me. Only quirk about the clock was the snooze function, which allowed me to sleep only an extra 4 minutes before the alarm went off again.
3. Maglite AA Mini Flashlight and Holster
Definitely a good buy for times when you need light (or torch) while walking around the city in the dark. It's produces a strong light considering the size, which is maybe about 1 foot in length. Heavy, but durable. I did not need to change the 2 AA batteries over 7.5 months. The light also comes with an extra lightbulb built into the flashlight in the case that the lightbulb burns out. The holster was unnecessary, in my opinion. I used it mostly when I was in hostels and didn't want to wake others up by turning on the main lights, and when I was trekking the jungles in Nepal at night.
4. Eagle Creek Travel Gear Hidden Pocket
I purchased two different hidden pockets, one that went around my waist and one that wrapped around one of my calves. These were great to separate your money and keep it hidden from potential pickpocketers. The calf pocket was more secure, but took longer to personally access. In addition, you could only wear the calf pocket with non-slim pants, and not in shorts, which you would probably be wearing throughout your visits to the hot and humid regions of the world. I ended up only using the calf pocket when I wore pants through the airports. I wore the waist pocket almost every single day of my trip and it provided easier access to my funds, while keeping it a little more safe than an ordinary wallet in my back pocket.
5. Pillow cover and duvet cover (from Ikea)
It is highly suggested for you to purchase any pillow cover and a cheap duvet cover if you plan on roughing it in cheap and potentially unsanitary hostels or guesthouses. I used an old duvet cover that I purchased for less than $10 at Ikea. That way, if you are ever in a potentially dirty room, sleeping in a bed with sheets that seemingly haven't been washed properly, you could always use the thin sheets of the duvet cover as a sleeping bag. The cover could also be used as a thin blanket if you ever need it.
6. Combination Locks
Combination locks are essential to keep your bags secure while walking through the streets or while they are in transit on a flight from one country to another. I particularly liked the MasterLock cable luggage locks because of the flexible cable, which made it easier to attach to my bags. It is a TSA-accepted lock, which means you can lock your bags and the TSA would not have to cut it open in the event that they need to search your bags. Note that if your main traveling bag is a 40+ liter backpack, chances are that there won't be a way for your to lock your bag with a lock. I used my locks mostly to lock my small day backpack and for lockers at hostels. It was also a little difficult to turn the combination dial on my locks after a while, which could be due to wear and tear or rust.
7. Compressible Storage Bags
I didn't purchase the storage bags shown above, but they provide a visual of the type of bags I am talking about. These bags are incredible at minimizing the amount of space your clothes fill inside an already small backpack. I ended up purchasing about 4 of these bags at a Japanese knick knack store in Malaysia. Put all your clothes in a huge zip lock back, roll the bag so that the air goes out a one-way valve on a corner of the bottom of the bag, and air does not reenter unless you open the zip lock bag to access your vacuum sealed clothing.
8. Amazon Kindle
Lightweight and compact, this device is essential if you plan on reading throughout your travels and intend on keeping your reading materials afterwards. I met a couple of backpackers who purchased books and wanted to keep them after reading them. They ended up carrying a stack of books with them throughout their travels instead of shipping them back, which created added weight to an already heavy backpack. I purchased the kindle with free 3G access, which made it simple to download any new books I wanted in most of the places I traveled to. I also tend to read more and faster when I read on a kindle, since I feel that I don't really know how far away I am from finishing a book, which makes it less likely that I will put it down from discouragement. My kindle unfortunately broke while I was in Nepal, and I had to resort to purchasing physical books, which wasn't a big deal, as long as I didn't keep the books after I was finished or I shipped them back home. Also, I found out through my travels that there were plenty of free book exchanges in hostels where you could swap a finished book for a new one, saving you some travel money.
9. Netbook
Essential for numerous reasons - to maintain contact with people back home, to blog, to store pictures, and to keep up with current events. While it was a slight pain to lug a mini laptop around, it was one of the easiest ways to stay in contact with friends and family through facebook or skype. Most of the hostels and hotels I stayed at offered free or inexpensive internet access, which made it more convenient and worthwhile to have a netbook with me.
10. Insect Repellent
Mostly for my travels in Southeast Asia, mosquitoes were prevalent in all the hot and humid areas. It is difficult to find a potent and effective repellent without the DEET eating away your sensitive skin, but I think it's better than constantly scratching bug bites throughout the day. No insect repellent will be 100% effective and you WILL get bitten numerous times, but I'll use something that will decrease the number of overall bites.
11. Unlocked International SIM Cell Phone
Having an unlocked international SIM phone was convenient when contacting couchsurfers and hostels/hotels. The price of SIM cards varied in different countries, but I usually spent anywhere from $5-$40 in each country to have the minutes/internet access for the duration of my stay. Having internet access made it easier to navigate a city as well, especially those uneasy moments when you first arrive after a flight or train/bus ride. The navigation also helps to reassure you that you aren't being scammed by a taxi driver who might drive around in circles, running up the meter.
12. Alice Park Flap Wallet
Not particularly necessary, but I loved my Alice Park wallet since I used it everyday. Slim and heavy-duty, it was easy to hold some of my cash and a couple of my credit cards in the wallet in the front pocket of my pants. I got it soaking wet a couple times, but it dried and was useful again and again.
13. Apple iPod Touch (16G)
Preferably with good speakers to block out any outside environmental noise...especially when you are stuck in a loud Karaoke bus in Thailand, or on any various night at a hostel where drunk backpackers are bound to come back during the wee hours of the night. Having my own music made me feel less lonely and more at home during the times I was by myself. Music was my companion.
14. iHome Rechargible Mini Speakers
These were stolen about 3 weeks into my trip, but I did get a chance to test them out for a month prior to my travels, and the quality of the sound in this little speaker was exceptional. I ended up having to boot up my computer to use the speakers while I was in my hotel room, which was a hassle. I am 100% certain I would have used these speakers throughout my trip had they not been stolen :(
15. REI MultiTowel Lite Large Towel
For the times you are not provided a towel from the hostel you are visiting. This towel is lightweight, thin, but super absorbent. Also, the material quickly dries, which is helpful when you don't want to pack a damp towel in your backpack as you continue along on your travels.
16. Flight 001 Passport Cover
Not really essential, but I found it nice to have a passport cover to keep your passport in good condition. It's better to have it in pristine condition when going through customs. Who knows what type of problems you could face with a wet, dirty, or torn passport in another country?
17. Johnson and Johnson Baby Wipes
I was especially thankful to have these when I was sweating my ass off in the humid and dusty countries I visited. A quick way to clean and cool off.
18. Zip-Loc Bags
Good for organizational purposes and also to pack your toiletries for your carry-on baggage at the airport.
19. Purell Instant Hand Sanitizer
You never know when there won't be water, soap, napkins, or a bathroom available...
20. American Apparel Tri Blend Tank Top
Probably the one shirt I used the most throughout my trip. It made me cool in every sense of the word :) The breathable and lightweight material made sweating tolerable, and it was one of the items that was quicker to dry when I hand washed my clothing. It lasted throughout my trip and I still wear it to this day.