King Kong Hostel is an ideal budget option located on the lively Witte de Withstraat. When we visited in May it had only been open for a month or so but things ran very smoothly. The manager was very friendly and helped carry our bags up the stairs and moved us to a different room with a double bed - which was a nice gesture. And when we somehow managed to lock our keys in our locker the staff arranged for a pair of bolt cutters within the hour, which we really appreciated.
They do a cheap breakfast which you can have downstairs or take away and there's also a nice movie room with cinema screen, sofas and magazines. As well as all that they have a free tea/coffee machine where you can help yourself, and if you want the good stuff they also have a cafe out the front with quality baristas.
One thing worth noting: King Kong is situated on one of Rotterdam's busiest bar/club strips so expect some noise during the weekend.