While you probably don't need most things listed below, many of these items make life on the road easier and comfortable.

Sleeping bag or sheets

If you want to sleep at hostels, you might be required to provide your own sleeping bag or your sleeping sheets. It is also useful if you plan to travel in a night train or if you find yourself sleeping in a train station, airport or wherever.
As sheets in concerned in most hostels (in hotels and pensions they are for free) you can rent them for a very cheap fee, so you can travel without the sleeping bag or sheet.
If you decide to buy a sheet there are two materials. These made from silk are expensive (three times the price of the cotton ones), but extremely lightweight and space conserving. Many people use them instead of the ones given in hotels and hostels. The cheapest ones are made of cotton, and will work just fine.

Travel blanket

Although a big towel can replace a blanket you could bring one if you are traveling to cold countries.

Pads and mattresses

Especially handy for people that will sleep at campings and want to do so over a smooth and comfortable surface.

Small first aid kit

For long stays it could be very useful to bring a first aid kit with all the basic medicines to deal with colds, small cuts or digestive problems. For example some kind of pain medication, a generic cold medicine, stomach-upset pills and skin patches for blisters and cuts (there are less common things that could be useful like band-aids, alcohol, condoms, antibiotics or insect repellents).
Before taking too many medicines, you should think where are you going to travel, the weather there and what kind of problems is your body used to. Try to narrow the odds as there are plenty of pharmacies all around Europe . If you're travelling with several people, divide the medications to each traveller to decrease the space taken up.
Specially for women, across Europe you will be able to buy all your normal hygiene products, but if you are more comfortable with what you are used to, bring them from home. The same goes for the contraceptive pill, which may be possible to procure in Europe with a doctor's prescription, but remains unavailable in certain countries.

Insect repellent

If you are attacked by insects or you are going to places with a big insect population you will avoid those little bastards. There are several skin lotions and electronic devices.

Sewing kit

If you think that it is useful buy a cheap one or make your own: wind some thread around a small piece of cardboard and insert a needle, extra buttons and a couple of safety pins.

Luggage locks

You can use them for ensuring the zippers on your backpack or to fix your backpack to somewhere. Better combination locks than those with keys. There are also a good variety of cables that have the same purpose.
More useful if visiting poorer countries they protect your things not only from thieves but also from curious at hostels.

Eating utensils

If you are eating much food from supermarkets and you are preparing the meals on your own take some cheap and light utensils to make eating easier. Although you can find utensils at hostels, it might be fewer or questionable looking. The minimum: a plastic plate and a plastic fork that will complement your water-bottle and your Swiss knife. Buy cans that can be easily opened or bring an opener if you aren´t sure of finding those cans.

Small foldable drinking cup

For drinking or brushing your teeth.

Food

If you have a low budget or you would like to eat you marvellous food of your country you could bring some food from there. Although you will save money be careful with the weight and how to carry it. You can buy daily some bread to eat your food.

Tobacco

Tobacco is really expensive in some countries, so you should take with you a great supply if it is cheaper in your country.

Odour absorber

It is not always easy to find time to wash your clothes. Place inside of your backpack to keep your clothes smelling fresh, as the odour absorbers not only mask odours, but absorb them also.

Long wire

Take one if you are sleeping in campsites in order to have electricity inside your tent (recharge cameras, mobiles, hairdryer, etc.). If you only need electricity for recharging electrical devices you can leave them in the frontdesk of the camping for a while. Be present while the recharging if the reception don´t seem a place to rely on.

Clothes line

You can take a clothes line to make easier drying your washed clothes.

Clothes pegs

A few pegs could be very useful at certain moments. If you are going to frequent campsites and you must wash your clothes they are almost obligatory.

Common soap or detergent

You can use that to do your laundry. The best ones are the concentrated laundry detergent or the liquid laundry detergent packets. However a bar laundry soap, or just plain regular soap or even your shampoo will fit perfectly the purpose. Remember that very little is required to do a sinkful of laundry.

Sink stopper

Very cheap and takes up very little packing space. Maybe you never need this. Besides, a sock stuffed in the hole will work pretty well.

Camera and film

Photography addicts can't live without one. Many things depend on the type of camera you bring. You won't need film if you bring a digital camera, but you might bring an adaptor plug set and a voltage converter if your camera needs to be recharged regularly.
If you prefer to carry your films from your country be aware of he damage that will be done to that film by passing it through airport X-ray machines, especially the higher speed films. For non specialists you won´t see the difference, but although it is said that X-rays won't damage your film, they will. So think about buying the films in Europe if the difference in price is not that high.
The digital camera user avoids this problem but must instead deal with batteries and chargers. Take into account the capacity of your memory card. If you get it full very early you can always try to download it and copy to a CD somewhere.
Don´t forget your tripod if you are a picture addict but think if it is worth, as it will take much space and weight.

Journal

For many people it is one of the best souvenirs you can bring back. If you like to write your experiences this will fill the boring moments (for example while travelling) and will allow you to remember those great moments that otherwise you will forget.

Reading books

They use to be heavy and bulky but can entertain during long travels or boring waits.

Calculator or currency converter

If you are bad at maths and don´t carry a mobile phone with calculator you can take one with you to make the exchange calculation or to follow your budget situation.

Addresses of family and friends

Useful especially if you plan to send postcards or letters.

Map or print out of my estimated route

A good option if you are visiting many places. You can use it to take decisions or write down where other places are and remake the itinerary while traveling. A nice way to have a free map is Internet. Go to www.viamichelin.com and configure the map as you want. When done press the Print Screen or Impr Pant button of your keyboard, open the Paintbrush, Edition, Paste and Save it. Then, with your favourite image editor you can cut the map, make it bigger and put it nice to print.

Phrase book or dictionary

Useful when you need to communicate with local people or to understand something.

Phone card

In Europe each country has very cheap international cards, but if you have better ones you can bring yours.

Sunglasses

Very recommendable during summer, as sun is really strong in some places.

Glasses and contact lens case

An eye glass case, contact lens case holder and mirror in one.

Sleeping well

Travel inflatable neck pillows are nice for long flights or long train and bus journeys. They are small and light.
If you are a light sleeper an eye mask or ear plugs are very helpful, as people turning the light on or snoring or making noise are common at hostels.

Small flashlight or lantern

It is handy at hostels and campsites. Use it to have light at night, to find the toilet, to read or to find the bed at the hostel dorm when lights are off.

Large envelopes

Use them to keep your bag light. If your travel is long you will accumulate tons of papers and brochures that you can send home with those envelopes. Again, you can buy them during the trip.

Rubber bands

To keep things and papers packed and organized. Many uses.

Woolen hat and glooves

In cold countries or in cold off-season they keep you warmer than any other piece of clothing.

Rain slicker

It keeps the rain and wind off, and it is very light. It acts as a shell.

Different articles to pack better or to organize the backpack

You can buy many different articles to gain space or organize better your items, although you can live without them.

Immersion heater

This is a valuable appliance to have boiling water in a moment. Nice to have a hot cup of coffee or tea, or even to cook simple things. It also could be used to kill all disease organisms if you suspect of some water. Small and light immersion heaters are inexpensive but you again should consider the voltage and plugs compatibility.

Mussic player and headphones

Many people enjoy having music at hand while they're abroad to fill the many hours spent traveling.

Laptop

They are rather heavy to carry around and you should have a really heavy reason to take it. There are many alternatives: from an internet cafe you can access to your computer's hard drive or your email from any computer connected to the internet. You only have to learn how.
If you decide to travel with the PC, you can get access to Internet at many hotels with connection in the room. If they offer the access to you through the telephone line, be sure that the number you mark is local. To call to a server located in another country will increase drastically the cost. Another alternative is to use the "Wi-Fi networks" without cable that exist now in airports, railway stations and other places. The Wi-Fi zones are in general indicated clearly. But your PC must have a special card for the connection without cable.

Voltage converter, plug adapter

Electrical power varies throughout the world. All Europe works with 230 volts and 50 Hertz. In Cyprus , Ireland , Malta and the United Kingdom they use square plugs with three pins, but, in general, the other european countries use plugs with two pins.
Therefore if you are an european citizen you will not have too many problems. If you come from another place and your electronic device is compatible with several voltages, you will only need the adapter plug sets. For all these reasons when you plan to take your hair dryer, your electric shaver or your mobile phone charger you might carry and adapter. Usually adapters can be bought in airports and tourist centers. If you don´t know yet what to do you can go to an electricity shop and ask an expert.
If you don´t have an adapter but need it ask the personal of your hotel or hostel, maybe they have some.
If your plug works with two holes but the socket to plug in has three, one solution is to introduce a plastic pen cap in one hole and your plug in the other two.

Pets

On July 2004 a new passport for pets took effect and it will facilitate to travel with them. All the cats and dogs must have one and it will be accepted in all the UE that they have received the antirabic vaccination. However, Ireland, Malta, the United Kingdom and Sweden also demand the test that the vaccination has been effective. The compulsory tests must be carried out at least six months before the trip to Ireland, Malta and the United Kingdom and 120 days before in the case of Sweden. An electronic microprocessor throughout the eight next years will be introduced to allow an easy identification of the animal. Meanwhile, a tattoo like identification can be used, except in Ireland, Malta and the United Kingdom where already the microprocessor is demanded.