Good things to take

Hardware

Avoid taking electrical appliances such as hair dryers and irons; they are heavy, take space and can be incompatible with local electrical systems. They are often provided in hotel rooms or can be borrowed if necessary.

Alarm clock, watch
Digital clocks are small and light. It is a good idea to set your watch alarm (if equipped), the hotel room alarm (if provided) as well as your own alarm for important events such as aeroplane departures. Always carry extra batteries!

Electric plug adaptors, transformers
Often hotel rooms have shaver outlets are compatible with North American appliances and can be used for battery chargers and other small appliances. If you must, take the appropriate adaptors for the area to be visited. Verify that step-up or step-down transformers are of sufficient wattage for the equipment you wish to use.

Pocket Stuff

Swiss Army knife, multipurpose tool
A corkscrew, can opener and bottle opener are essential. Scissors are also useful to have. Don't carry this item in your pocket or carry-on luggage when boarding transportation. It is often seen as an arm and is confiscated by security personnel. The storage rooms of airports that have become Swiss Army knife dungeons are really amazing!

Compass
A small compass that attaches to a key chain or a zipper tab can be a helpful item, to re-orient yourself on a hike or even in a large city.

Miniature Flashlight
A krypton or halogen lamp powered by a couple AA cells can be extremely bright. Always carry extra batteries and bulbs!

Candle
To provide light in case of electrical failure as well as heat (image being stranded in a car in a snowstorm!).

Matches, lighter
Matches should be waterproof safety matches. Lighters that have piezo ignition devices are preferred as they are less vulnerable to humidity.

Pens
Always needed! Inexpensive but stylish pens, not locally available, make good gifts.

Small notebook
A notebook should be small enough to fit in a shirt pocket. There are useful for many things: addresses, expense account, diary, notes...

Handy to Have

Rubber door wedge
A wedge to block intruders, this is essential for a lone woman or while taking a shower.

Rubber bands
Innumerable uses. Maybe you can't imagine them now but when you don't have any!!!

Glue stick
Innumerable uses. Idem above!

Felt-tip marker
Many uses: label envelopes and luggage, make a signs...

Envelopes
It is often easy to find paper to write, but small envelopes may not be as readily available as stamps and a post box. Self adhesive envelopes are preferable because they do not accidentally seal closed in in high humidity. You might pre-address envelops for letters you foresee writing, before you leave home. Choose a size large enough to enclose postcards as mail in envelopes is generally given higher priority treatment than postcards. Large envelopes made of tyvek are compact, light weight (type used by express package services) and can be used to mail things home.

Zipping plastic bags
Several large and small bags are always useful on any trip.

Garbage bags
Garbage bags can be used for everything from carrying laundry to emergency rainwear and as bivouac.

For Comfort

Inflatable travel pillow, earplugs, eye mask
These items are helpful in order to get a rest in a hostile environment .

Hostel sheet bag , sleeping bag, bivouac sack
These may be essential items for Youth Hostels, trekking, and less than clean hotels.

Dark glasses, retainer, case
Please don't forget the sunglasses, essential everywhere: in snow, sandy beach or desert, on water... Glasses retainer cords are requisite if you are going to be involved in outdoor or sport activities as well as a good case for protection. Don't lose them!

Spare glasses, copy of eye glass prescription
Don't risk handicapping yourself by lost or broken glasses or contact lens.

Gadgets

Camera, flash, film, batteries, filters, lens cleaner
In order to avoid errors, it is best to try to use film of the same sensitivity. Despite statements to the contrary, airport X-ray inspection does damage film, especially film passed many times through X-ray machines in the course of a long trip. Effects of X-rays are cumulative. It is best to transport film a lead-lined film shield envelopes or cartridges. Film should be carried in carry-on luggage and ask for manual inspection. Film in checked luggage is more likely to be damaged because checked baggage is inspected with higher power X-ray machines. Always take sufficient film for your trip and as well as extra photo batteries! Specialized batteries for cameras may not be available or be extremely expensive abroad.

Video camera, cassettes, batteries, charger, filters, lens cleaner
Don't forget to take your battery charger and any accessories when leaving home and hotel rooms. Please note: Certain types of batteries (even 9 volts transistor radio types) are very rare and expensive in certain countries. Authorities may confiscate them (probably for sale on the black market).

Cellular-phone, pager
Cellular-phones and pagers can be useful devices when travelling but telephone systems in different parts of the world function on different standards. Your devices may not be compatible. If you require a cellular-phone or pager in a foreign country, they are often available for short-term rental at airports.

Miniature tape recorder
A mini-cassette tape recorder is very useful for recording notes and interviews while travelling.

Radio
Miniature all-band radios may be useful for getting news and for entertainment when travelling in certain areas. Local radio service may not exists or be politically controlled. Be warned that shortwave radios have been confiscated by authorities in certain countries.

Walkman, CD player
Miniature portable players provide agreeable entertainment, on long trips, while waiting for connections or while staying in very isolated areas . A plug doubler is an interesting accessory that will enable you to share your pleasure with someone else who has a headset. One must be aware that these devices are very desirable, are sought be thieves and have been confiscated by authorities who will give the strangest justifications. In some countries batteries are rare and extremely expensive. You might not lose your player but to might lose your batteries!

Calculator
Solar powered calculators eliminate battery problems. Special calculators that do currency conversion can be useful for the traveller.

Pocket translator
An alternative for phrase books. They may be faster to search but have a limited utility.

Binoculars
Binoculars can be an interesting accessory for someone involved an outdoor activities, or as a spectator at the opera, theatre and sporting events. Extremely compact models are available; seven or eight power magnification is generally recommended.

Entertainment

Small musical instrument
Playing a small musical instrument such as a harmonica or flute can provide an interesting cultural exchange.

Reading material
A good paperback novel is often enjoyable to read during long waits for transportation or while on the beach. Books can also be left behind as appreciated gifts.

Playing cards, travel games
Playing cards and travel versions of games like scrabble and chess can keep children, and adults alike, occupied while travelling.

Gifts

It is often agreeable to have small gifts to give as appreciation while travelling. Items such as smart looking inexpensive pens and pantyhose can be given as gifts to a household staff, were these items maybe rare or expensive. Leave them on your pillow when leaving. Cassettes and small books that express your "home" culture are nice gifts where social contact is important. Sometimes foreign citizens have a high appreciation for your country of origin. Small pins or crests with your national flag can be an appreciated gift. And finally don't forget the 'used' gifts such as pieces of clothing, the novel you brought along... they many be more greatly appreciated than you could ever imagine!