The group of hostels known as www.GeneratorHostels.com gets a good review in an article in the Huffington Post (www.huffingtonpost.com.) They have properties in several European cities, and feature hostels with modern design and rates starting as low as 10 euros per night for a bed in a dorm. They also offer single and double rooms at higher cost.
If you've read previous blog posts you know I am not generally a fan of hostels. However, hotel prices in Europe can be quite high, and I would consider trying a hostel again in a pinch. From the photos on their Website Generator Hostels look clean and attractive, but their dorm rooms can hold as many as eight people in bunk beds.
It seems to me that hostels are more geared to young people than to others, and the photos on the site certainly indicate that. In addition, some from what I have read have a hard-drinking culture that would not appeal to me. If you check hostel ratings, apparently a high rating for "fun" usually corresponds to heavy drinking.
Generator Hostels has two properties in Berlin, one in Mitte and one in Prenzlauer Berg. Rates this summer are considerably lower in Prenslauer Berg, a district of the former East Berlin just next to Mitte that is a little more green and quiet but equally hip.