Cantabit vacuus coram latrone viator.
Travel light and you can sing in the robber's face.
What To Pack
Things that must be plugged in (see also "What About Batteries?", below) are in conspicuously short supply on my own personal packing list; they add bulk and weight rarely compensated for by usefulness. Try hard to eliminate them.
If, however, you just can't live without your electric shaver (a small bottle of shaving oil and a razor are much more appropriate), or your hair dryer (think about towel/air drying, or even a more travel-friendly hair style), don't forget that electrical power varies throughout the world.
Plug adapters and voltage converters are available for all of this, of course (Walkabout Travel Gear is a particularly well-equipped and helpful source of such devices); also check the "Destination" section of the TraveLinks page for an excellent information site). Remember that, when using converters, you must also be concerned with power (wattage) ratings. And some devices — often those with motors — are sensitive to the AC frequency, which cannot (in any practical manner) be converted.
Plug adapters can be found featuring all sorts of ingenious constructions, such as the "Tripshell Universal Travel Adapter" (pictured at left), which provides configurable plug adaptation for some 150 countries worldwide. It incorporates a 6A fuse (which means it will handle 660W @ 110V, 1440W @ 240V), L-N surge protector (to isolate sensitive electronics from potential power spikes), 110/230VAC voltage indicators, and child safety shutters. These can be a convenient solution if you're travelling to many parts of the world on a single trip, but for a modest number of destinations, simple single-solution adapters will be smaller, lighter, and considerably more reliable.
Many products (especially modern electronic devices such as computers, cameras, and cellphones) are available with "universal" power supplies or multi-voltage settings; you'll still need plug adapters, though. And remember: if you carry electrical devices that must be plugged in, you should be prepared for a shortage of available outlets (often in limited-supply/high-demand locations such as airports and hostels); consider bringing along a cube tap or the equivalent, to help both you and your fellow travellers.
If you choose to travel with a device having a 120/240V switch, get into the habit of switching it to 240V when you pack it: should you mistakenly plug it into a 240V socket when set to 120V, you will have an ex-device.
The various cables associated with electronic devices (some of which are considered in the Specialty Items section of the OneBag packing list) are often the weakest links in the chain; make sure you travel with undamaged, high-quality versions (Anker has achieved renown in recent years as a supplier of robust, inexpensive cables). Cables can also be sources of unnecessary bulk and weight, so only carry lengths sufficient to your needs: instead of a three-foot cable, perhaps a four-inch version (such as Anker's tiny USB/Lightning cable, seen here) might be sufficient.
If you've become wedded to a smart phone or other portable device, and your travels take you to places where electrical outlets are in short — or nonexistent — supply (including long flights and the like), you need to think about carrying a spare battery, something more easily managed with some devices than others. One well-regarded solution is the Anker® line of portable, rechargeable power packs; their mid-range (10,000mAh) model pictured here is a particularly appealing blend of storage capacity with modest weight/size, capable of charging a USB-connected device at up to a 2.4-ampere level. It's a decently compact 3.62×2.36×0.87 inches (9.19×5.99×2.2cm), and weighs a not-too-intolerable 6.4 ounces (181g). Larger and smaller models are also available
Many modern, low-power electrical items can be operated and/or recharged from USB ports, reducing the need to carry separate (heavy, bulky) converters. If you lug a laptop, you can plug them in there, though many modern televisions come with USB ports as well, so check the one in your hotel room. Alternatively, carry a tiny USB wall charger: I like Aukey's excellent travel version, as pictured here, with its dual 2.4A outputs (each with AiQ support), and a very convenient folding plug to eliminate snags when in your bag.
Of course, you can best eliminate electrical hassles by eliminating the gadgets.
Even if a hair dryer does make a great sock dryer.
So … No Electrical items?
You take what you need
And you leave the rest
Small, lightweight immersion heaters — like the Franzus IH100 model pictured at left — are inexpensive and available in dual (120/240) voltages, though you may still need a plug adapter. Alternatively, you can buy such heaters locally in most developed countries.
You may read deprecating remarks about the reliability of such heaters, mostly from those who don't understand that they are designed to be plugged in only when the heating element is fully immersed in water. So make it a habit: immerse before plugging in, and unplug before removing. Always. Otherwise they can burn out in a flash (even the ones that claim to have built-in thermostatic shut-offs)!
What About Batteries?
Battery-operated devices are more acceptable for travel needs than those requiring mains connections, but they are not without issues of their own: you may well need to replace (or recharge) them throughout the course of your trip, and your travel planning should include considerations for same.
Batteries come in many different sizes and voltages; the most popular (1.5V) batteries for consumer products are pictured here. AA-sized batteries, like soap, are pretty much ubiquitous; consequently, you can reliably depend on being able to replace these wherever you travel. If your devices use anything other than AAs, you need to give serious thought to carrying sufficient spares.
One option, if you expect to go through a lot of batteries (such as for photography) is the use of rechargeables. Eneloop Pro batteries (made by Panasonic) are widely considered the best rechargeable AAs, with regular Eneloops running a close second; the Pro version holds a greater charge, but can be recharged fewer times than the standard version (most people, however, won't hit the 500 limit any too soon, and more power is better than more recharges, in my experience).
Such a course will require, in addition to the specialized batteries themselves, some sort of recharging device. Panasonic makes a fine, inexpensive battery charger that plugs in directly (no power cord); if you want a more sophisticated model (which will recondition older batteries as well as recharge them), go for the La Crosse charger, but it's twice the price, considerably larger & heavier, so less suited for travel needs. Both of these chargers are "smart", in the sense that they tailor the charging rate to each individual battery, and stop charging when it has reached capacity (you should choose no other type of charger), and they will both work on worldwide electrical systems; again, though, you may still need a specific plug converter for your particular destination(s).
Finally, give some thought to lithium batteries, which offer significant benefits to the traveller. They are much lighter in weight than both their alkaline and rechargeable counterparts, retain their charge for many years, don't leak (like alkalines can sometimes do), and perform better in extreme temperatures (both cold and hot).
Recently, flashlight manufacturers have discovered the benefits that accrue from matching modern LED technology with the high energy-density 3-Volt CR123A lithium battery (pictured here in comparison with an AA), also sold under a variety of incorrect designations, such as CR123, 2/3A, 123, 17345, 16340, 6135-99-851-1379, and even just "camera battery". Consequently, the very best small LED lights now use this approach almost exclusively (and this is what I have come to prefer).
Lithium cells are often overpriced when purchased locally, one or two at a time. When ordered in bulk from large suppliers, though, both CR123A and AA versions can be very reasonable in cost (and unlike other batteries, have a shelf life exceeding 10 years).
Just remember that it can be difficult to find CR123As out there on the road, so if you decide to use them, plan accordingly.
And yes (in case you were wondering), there are rechargeable lithium batteries as well, though these require specialized chargers.