Hostels & Backpackers
Hostelling International is the world’s largest accommodation provider. Most youth hostels accept reservations. Almost none have age limits.
IYHF (International Youth Hostel Federation) publishes two annual directories of Hostelling International hostels: Volume 1: Europe and the Mediterranean – Volume 2: Africa, Americas, Asia and the Pacific.
Private Hostels & Backpackers
Hostels.com is the most complete for hostels worldwide, with also many private and independent hostels.
Hostelworld.com is a web booking service for private hostels. It lists fewer hostels than hostels.com, but has a wider range of budget accommodation, campsites, etc…