If you bring everything on this list, you’re doing it wrong. Some things you obviously need — some might just be fun to have if you have the extra space in your bag. This list will forever be a work in progress. Feel free to add your own suggestions in the comments.

Absolute Essentials – You won’t make it far without these.

• Passport (with visa if the country you’re going to requires one)
• Photocopy of your passport (write a few emergency contact numbers on the back side)
• Driver’s License / Photo ID
• ATM & Credit Cards – Be wary of international transaction fees imposed by your bank.
• Insurance Cards
• $100 (or Euro) bill for emergency use only – Kept in a separate part of your wallet or elsewhere hidden.
• Contact Lenses and case and/or glasses
• Clothes for the plane – Dress in layers. Wear your jeans if you’re bringing them and other heavy or bulky items that will reduce your luggage weight or size.
• Health-essential items, i.e. prescription medications, insulin if you’re diabetic, Epipen if you’re allergic to stings, etc.

Luggage – Name, address and contact info on the inside and outside of each bag.

• Carry-On Backpack – All valuables should go in here when on the move.
• Full-size Travel Backpack – Assuming you plan to check a bag.
• Day Pack – Goes inside or piggybacks onto your full-size backpack.
• PacSafe Travelsafe – For keeping your valuables secured in your room.
• Packing Cubes – Some people swear by them. (I don’t.)
• Compression Bags – Helpful for compressible  items like fleece, not so much for anything else.
• Bag Locks – TSA certified. Or just use zip-ties. Never put anything of value in a bag that will be out of your sight.
• Money Belt – Keep your passport on you at all times, at least while flying. Alternatively, look at arm wallets, leg wallets and shoulder wallets.
• Compact Umbrella – Kept in one of the external pockets of your backpack.
• Zip-Lock Bags – Useful for keeping things dry inside your bag.
• Dry Bag – If you’ll be traveling on boats and there’s a chance your gear could take an unexpected dip.

Geek Gear – All such items should generally go in your carry-on bag or on your person, to avoid theft.

• Smartphone – Phone, camera, video player, music player, web browser, GPS…
• Laptop / Netbook / Chromebook – If you need to get any work done while traveling, something with a physical keyboard is pretty much essential. Plus the larger screen can be nice for watching movies if you’re stuck in bed on a rainy day.
• Tablet – Personally, if I’m going to carry extra weight I’d rather have a laptop, but maybe you’re a tablet person.
• Kindle / eReader – If you plan on doing your reading by the beach or around the pool.
• Digital Camera – If your smartphone camera just doesn’t cut it for you.
• Mini-Tripod – If you want to get artsy with your camera.
• Travel Router / Repeater – Useful for creating a hotspot or passing on weak Wi-Fi signal. Choose one runs on USB power so your phone charger can do double duty.
• MP3 Player – A small, clip-on MP3 player can be great for the plane or the beach, and less of a concern if you lose it.
• Chargers – For all the above. Try to stick with gadgets that charge via USB, so you only have to bring 1 or 2 chargers.
• Power Cables – For all the above. Aim for double duty cords for USB-powered devices. I tend to go with one micro-USB cable that’s long enough to charge my phone or Kindle while using it, and one very short cable (to save on weight) for charging and as a backup.
• Noise-Isolating In-Ear Earphones – They block out sound on flights, don’t require batteries, and can double as earplugs.
• Airplane Headphone Adapter – So you can use your earphones with the airplanes in-flight entertainment system.
• Headphone Splitter – So you and your travel buddy can enjoy the music or movie.
• Travel Speaker – Great for tunes at the beach.
• LED Flashlight – Get a bright one that runs on a single AA or AAA battery.
• USB Battery Charger – If you’re bringing devices that run on AA or AAA batteries.
• Watch (with alarm) or Travel Alarm Clock- If you don’t trust your smartphone’s alarm clock.
• Extension Cord – A short one, for those inconveniently placed outlets. Get one with a 3-way splitter on the end.
• Surge Protector – Essential in developing countries with unstable electricity.
• Plug Adapters – Assuming you’re headed to a country with different electrical sockets.
• Memory Card Reader – To transfer photos from your camera to your laptop, if it doesn’t have one built in.
• USB Flash Drive – In case you can’t backup your photos and videos online.
• Spare Battery / External Battery – For your digital camera or smartphone, in case you have to go a couple days without electricity.

Daily Necessities – Things that I’ve found I need every day.

• Sunglasses – Best bought from a reputable source so you can be sure they block 100% of UV rays.
• Padlock – For your room or locker in hostels or budget resorts. If keyed, get one where the key has to be in the lock when locking it to prevent locking yourself out. Avoid common circular-dial combination locks which are easy to defeat.
• Keychain LED Flashlight – Because you never know when the power might go out in a place like India.
• Sarong – A thin piece of cloth about the size of an over-sized bath towel. Functions as a towel, a beach blanket, a scarf and even an outfit if you’re a female (or a daring male).

Regional Necessities – Things you may want if you’re going to a developing country.

• SteriPEN UV Water Purifier – Purify water right from the tap.
  
• Mosquito Net
• Anti-malaria Medication
• Toilet Paper – Not a “given” in countries like India, so you may want to bring half a roll in case your bathroom doesn’t have any. You can generally still buy it locally if you’re in an area frequented by tourists.
• Swim Goggles
• Dive Mask – If you like to snorkel but have trouble finding a dive mask that fits your face, consider bringing your own.

Footwear – Shoes are often big and heavy. Ideally you only want to travel with two pairs: one on your feet, one in the bag.

• All-Purpose Shoes – Try to find shoes that are both comfortable to walk in and will look acceptable going to a nicer restaurant.
• Shoe Inserts – For better arch support.
• Hot-Weather / Beach Shoes – Sandals, flip-flops (a.k.a. thongs in some countries), Sanuks.

Clothing

• Functional Pajama Pants – ones that can also be worn for other activities, i.e. yoga.
• Pants / Jeans – Ideally with deep front pockets so that items are harder to pick. Never put anything valuable in your back pockets.
• Shorts – Again, ideally with deep front pockets.
• T-Shirts / Tank Tops – For warm climates, of course.
• Long-sleeve Tops – I prefer zip-up track jackets to quickly adapt to changes in temperature.
• Socks – Try merino wool socks from Smartwool or Darn Tough for great socks that don’t stink.
• Underwear – Keeping in mind that packing space is at a premium (and that applies to both girls and guys).
• Swim Trunks / Bikini – Whatever you wear when you take a dip.
• Belt – There are money belts that look and function like actual belts, if you want to have a secret money stash.
• Hat / Cap – For sun and wind protection.
• Ultralight Windproof and Waterproof Jacket – If there’s a chance you’ll encounter cold, wet weather, a thin jacket that can keep the wind off of you and keep you dry can be a life-saver.

Clothing – Female Specific

• Bras
• Dresses / Skirts
• Jewelry – The less, and the less expensive, the better.

Cold Climates

• Winter Jacket – You should be able to wear this on the plane, so you don’t have to pack it. Get a compressible one that goes in a stuff sack if you’ll be traveling in a range of climates.
• Cap / Beanie / Hat – In cold weather, a lot of heat is lost through your head.
• Lightweight Gloves
• Long Underwear – Could double as pajama bottoms.
• Scarf – Or something that can be used as a scarf, like a sarong.

Toiletries – Keep in mind that most of these items can be purchased locally if you want to save on pack weight.

• Insect Repellent – Free free to bring your “natural” stuff, but be prepared to buy some stuff with DEET locally if it doesn’t work. Look into permethrin for your treating your clothing and treat key items before your trip.
• Sunscreen – Even just a small bottle so you’re prepared for your first day of sun until you can buy a big bottle. Note that in remote areas of tropical countries where only tourists buy it, sunscreen can be exorbitantly priced.
• Toothbrush and Toothpaste – Easy to find and often cheaper in developing countries.
• Dental Floss – Also useful for repairing gear.
• Comb / Folding Brush
• Hair Product – Pomade, forming cream, etc. Keep in mind the humidity if you’re going to the tropics.
• Soap – Kept in a travel container. Body wash is heavier and not necessarily better for your skin if you buy a quality bar of soap.
• Shampoo / Conditioner – Put into travel size bottles. You may only need half as much conditioner as shampoo.
• Nail Clippers – If you’ll be gone more than a week. Can also be used to cut string and open bags.
• Shaving Razor – And enough replacement cartridges for the duration of your travels.
• Shaving Cream / Gel / Oil – If you insist, but I get by using only soap lather in the shower.
• Lip Balm
• Spare Contact Lenses
• Contact Lens Solution – Plus an extra contact lens case.
• Cotton Swabs – a.k.a. Q-Tips
• Condoms or other contraceptives

Toiletries – For The Ladies

• Make-up, make-up remover, make-up removal pads – Think minimal.
• Nail polish, nail polish remover, nail file – I’d highly encourage you to leave this at home or buy it locally.
• Tampons or a Moon Cup
• Pregnancy Test Kit

Handy Extras

• Swiss Army Knife or Multi-Tool – Some people can’t leave home without one, but they’re heavy and a target for thieves in your checked bag — and you can’t bring it carry-on because of the knife. If you bring one, keep it small, light and cheap. 
• Utility Cord – For hanging hand-washed clothes out to dry.
• Pillow Case – For a laundry bag which can double as a pillow in a pinch.
• Silk Travel Sheet / Sleeping Sack – When sleeping in budget accommodation in the tropics, it may be all you need.
• Pen – Black or blue ink. Other colors may arouse scorn from immigration officials.
• Plastic Spoon – Especially if you’re the type to buy yogurt from a local grocery store for a snack.
• Travel Guidebook – Generally I prefer doing my research online in advance and taking notes.
• Rubber Doorstop – For some added security to keep the door closed.
• Micro Compass – Because smartphones don’t always work, and finding north is tricky when it’s cloudy.
• Sewing Kit with Safety Pins – For light repairs.
• Small First-Aid Kit – Band-aids, disposable thermometers, Tylenol, Aleve, Imodium AD.
• Small Survival Kit – Fishing line, fish hooks, duct tape.

Fun Extras

• A Paperback Book – It never needs charging, it won’t break, and you can probably trade for another book.
• Frisbee (a.k.a. flying disc) – A regular item in my beach bag. Standard 12-inch discs are a bit cumbersome for travel, so consider something smaller.
• Hacky Sack (a.k.a. footbag)
• Cards – Note that card playing can be illegal in some countries.
• Chess Board (Travel-Sized) – Arguably the most popular and well-known board game in the world.
• Journal – For jotting down thoughts.
• Canadian Flag Bag Patch – If you’re Canadian (or an American preferring to appear as a Canadian).
• Vegemite – If you’re Australian.