There are a variety of items I consider a 'must-have' when backpacking in Europe or otherwise. They range from money belts to clothes pins, to power converters.
Here is a list of items I never travel without:
Guidebook, Money Belt, Swiss Army Knife, Convertable pants, Photocopies of documents, A pen and a note book
Guidebook - Your guidebook is your best resource for finding important information such as places to stay, eat, local customs, everything you need to know about the city or country that you're in. Click the link above for some great guidebooks.
Money Belts - Money belts are great to store your passport, money, and other essential documents. They fit snugly under your clothes around your waist, and there's no way someone can grab it without you noticing. Next to a backpack and guide book, I'd say it's the most important item you need with you. Click on the link above to view some of the different kinds of money belts.
Swiss Army Knife - The best multi-purpose tool available. Keep it handy. I use it for everything from slicing food to clipping my nails, it's useful for so many things.
One pair of convertable pants / shorts - One of the greatest inventions. Shorts for the hot summer days, and pants for the cooler evenings. Also, many religious sites require you to be wearing pants so it's a great quick fix.
Keep it on hand for writing down contact details of people you meet along the way and for recording your experiences. It's rewarding to read through a travel journey when you're back home and re-live your experiences.