Not so much a hostel, more a way of life, and a stalwart of the Interrailing scene. Young (they don't accept guests over 40 - or under 16) backpackers flock here from around the world, as much for the social life as the accommodation. You can see why: the hostel organises walking tours and in-line skating for free, and there are regular parties and cheap beer.
There are branches near the Vondelpark and on the beach at Noordwijk-aan-Zee; the latter is open all year but comes in to its own in the summer, when water sports, beach activities and barbecues are the order of the day. A free shuttle bus ferries guests between the beach and uptown hostels for free.