Other useful (and perhaps less-than useful) equipment
Everyone has an opinion on what to carry. My innate nature is to be a walking hardware store - an atavism of both my military and civilian life. I need to be self-sufficient. However I also like to be able to move without prolapsing my posterior. I take the view of lining up all I would like to take, then probably bin 2/3 of it. This is what you should consider, in addition to the base kit discussed in the previous sections.
Notebook, writing and map-marking materials, romer/ruler, protractor. Self explanatory. Navigate and Plan and document your progress, scribble any necessary calculations. Write love notes to pass to the cutie at the back of the group - whatever else you fancy.
Backup compass. Compasses can get lost and damaged. I carry a light baseplate compass as a backup.
First aid kit. Pre-packaged first aid kits contain a lot of unnecessary stuff, and are usually expensive. It is preferable to build your own.
Fire lighting kit. There is nothing worse than having a stove and not being able to light it. Matches are a bit too old school - I just carry a couple of disposable lighters. However I also carry a fire-lighting steel and flint, and a few segments of wax candle. Candles are great for extending the length of your lighter when lighting damp tinder.
Aide-de-memoire/VueeTuee. A VueeTuee is a great way of storing key navigation tables and basically any information you will need on the trail. Remember you won't have the internet to quickly look up what time the Dippers rise, when sunset is, what distance hand angle measurements correspond to, which plants are good to eat, and the planning ranges for direct and indirect fire. OK, maybe not that last one.
Pace counter. Keep one conveniently attached to you. You soon get into the habit of clicking according to your pre-calibrated pace count.
Knife. A sharp, strong, folding knife is an invaluable tool.It is wise to carry a small sharpening stone as well. Large rambo-style knives have their place, but only if you are planning to do some serious construction with it. Other than that, they are not worth the weight. Even in the army I never needed to use my combat knife. It was just a pain in the proverbial to carry when my small clasp knife did everything I needed. If you do need a big knife, ignore the Rambo movie hollowed handle type. Get a strong knife with a tang (the metal blunt part of the blade) that passes right through the handle.
Sunglasses. Essential in open country and particularly on snow.
Sunscreen. Sunburn is a self-inflicted wound... Protect yourself
Insect repellent. I prefer hard-core DEET based products. I've worked in a lot of areas where mosquito-borne diseases such as Dengue and malaria are rife. Aedes egyptii and Anopheles spp. mosquitoes eat anything with less than 80% DEET for breakfast. I've had malaria a few times before - it ain't pleasant.
Flashlight. I carry a mini-maglite.
Others carry headlamps such as the Petzl type, which are perfectly serviceable too. My view on using lights is that they are just for admin stuff like cooking, fiddling around with stuff and so on at your campsite. I believe they should not be used for navigation. Opinions may differ but as I see them flashlights narrow your focus to a very narrow beam at the expense of your night vision ability to see the larger picture of the terrain. Needless to say in the army we never used white light on night patrol, and we seemed to get around just fine. That being said, occasional nav checks on your nav data sheet and/or map will require light. For this, filtered light is fine. You should use two different filters. In the old WWII movies, red light was de rigeur and indeed it is very useful. Your night vision doesn't take as much of a hammering. However, what colour are contour lines? They are invisible under red light. This is not very useful. Green light similarly protects your night vision but allows contour lines to be seen. On the down side, vegetation marks can't be seen. I consider contour lines more useful. So, either buy some expensive filters for your lights or use a couple of layers of cellophane with a bit of insulation tape to make a lens cover instead.
Batteries Self explanatory.
Garbage bags These are useful for waterproofing your spare knickers & socks, as an emergency poncho (no, not kidding), solar still, water collecting. Oh, and they can hold garbage too.
Satnav. Wrist models are good if you can afford them. I carry a basic Garmin Etrek. Make sure you have batteries, and really it should only be there as a Nav Check. It defeats the point of hiking if you merely follow the arrow like a video game.
Cell phone. Much as I hate the things, they are a useful safety item for obvious reasons. Pre-program emergency numbers into them. Note that most (if not all) SIM cards have 112 (Europe) and 911 (USA) programmed in them. In some places, especially those with well developed rescue services such as Andorra for example, may have Apps that can feed location information to rescue services. One example is Alpify. If you have a smart phone (I just have a $10 cheap basic thing, smartphones scare me), then it is worth checking these out. Additionally, many smart phones can provide your position to some degree. Brotherton provides some excellent information on uses and procedures for cell phones.
Digital camera. Aside from recording your trek for posterity, they are useful for snapping landmarks along the way and, importantly, the view you should see on the way back. Notwithstanding the fact you should be making notes or sketches as well. They are also useful for the coroner when you take a selfie too close to cliff edges or next to grizzly bears. Gotta love Darwin...
Binoculars. Always useful for resections, especially as it gets darker. Also, being a bit of a birdwatcher, kind of necessary. I have a set of small 10x magnification ones (not this one - image for illustration only, haven't used this model) that fit neatly into a pouch.
Multitool/pliers. I'm always in two minds with these things. I use mine all the time in the boatyard and on boats, and they're pretty useful out bush. But they are heavy and overloaded with too much stuff I'll never use. A (relatively) basic model like the Wave multitool is worth considering. At a minimum I carry a pair of light pliers with wire cutters. Read the reviews - some models and brands are better than others. Many reviewers warn folks away from Swiss Army models as too flimsy. But to my mind, keep the gadgets to a bare minimum. If you really need that bit that takes stones out of horses hooves, get a purpose-built one.
Do I really need it?
Walking stick/ski pole. Whoever came up with the idea to make and market these must be laughing all the way to the bank. The more astute of you may have detected that I think they are kind of silly. My reasons are that, well, apart from never having any need of them, I prefer to have my hands free for map, compass, rifle and so on. I also have a knee jerk reaction that the marketing of expensive sticks has somehow led everyone to think we can't actually walk without them. I'm also sick of tripping over people's sticks in supermarket aisles. OK, they have their fans who I don't mean to insult. They can be useful tools for measuring heights, and if used properly they might help a bit going downhill (see discussion by the siteowner of sectionhiker.com). But they just seem to contribute to bad posture going uphill and detract from observing around you. But, hey, that's just my opinion. If you want to carry extra stuff for no reason, go for it.
Personal locator beacon. My experience with these is largely restricted to marine EPIRB's, which is the first thing you grab if your boat starts to sink. PLB's are basically a panic button that will transmit a locatable signal. Are they worth the weight? That will depend on your task objective.
Avalanche rescue kit. If you are hiking above the snowline, cross-country skiing, or otherwise out on dodgy piste (so so speak!), a readily-accessible rescue kit of transceiver, shovel, probe is wise to take. The transceivers are units that can both transmit and receive signals in the event of burial by avalanche (transmitting), and searching and probing (receiving) for the avalanche victim. They save lives. If you need this sort of equipment, get proper training in their use and ensure you base-test and check according to manufacturers instructions.
Work of the devil...
Selfie stick. Selfie sticks or 'wands of narcissm' are the height of vanity and stupidity. Why go to a cool place just to take 20 photos of yourself trying to attain that supermodel look? Surely you must know what you look like by now? I personally have to remind myself to take photos full stop - I'm too busy looking and experiencing, rather than just recording it for very uninterested folks to look at later. See my comments on photos versus sketching if you need to truly observe things. Face it, travel photos are particularly boring when they just consist of your own face. That's just my opinion, and in no way reflects my jealousy that when I was born, the doctor saw my face and slapped my mother.