Simple but very clean hostel near the airport (8 min from Barajas station), good option to crash in if your flight is departing very early the next day (or arriving very late). The room has its own bathroom.
The concierge was friendly and helpful (good service, he managed to find and lend me an electric plug adaptor which I really needed for my cellphone charger although I was checking-in at 2am).
Good quality for that price, I recommend it.