I had the best stay of my life at the Goodbye Lenin: Let's Rock! Hostel in Krakow, largely due to the group of people we met there. Two Australian brothers had a charango (a South American stringed instrument) and a baby grand piano purchased from a flea market in Budapest, and we had many vodka fuelled singalongs in the hostel common room.
This comes from a night where the hostel gave us all free shots, and we stayed up until 6 in the morning singing, talking and drinking, making as much noise as we wanted because everyone in the hostel was gathered in the one room.