Hi everyone 
I would like to ask you if you know some cheap hostels in Europe or places/sites where to find them. I know hostelsword and airbnb but you know there is always something interesting that you don't know .
It would be good if price for the place would be no more than 15euro.
Rwgards
Try hostelbookers and couchsurfing. These might help:
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http://thesavvybackpacker.com/hostels-in-europe-guide/
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http://www.independenttraveler.com/travel-tips/hotel-and-b-and-b/ditch-the-hotel-10-cheaper-ways-to-stay
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I am not sure what you have on mind. Could you be a bit more specific? Europe is a very very large place.... For example if you find a 10 euro hostel in some kind of one horse town with nothing to see and do while you are after nightlife and sightseen plus you need 18 hours traveling to reach on trains and buses, would you still go???
Can you mention where exactly in Europe you will be going? I stayed at a pretty cheap hostel in Amsterdam called the Flying Pig. It was a party hostel though so it really depends on what you are into. Couchsurfing is really ideal if you are looking to save money. I did it in Bern, Switzerland and also Madrid, Spain and loved it. I often use Offhotels when I can't find Couchsurfing hosts and find that the prices are really good.
-Shannon