Major hostel organization that offers membership. Membership entitles the cardholder to get discounted rates at an "HI" hostel, although such is not necessary to stay at such a hostel. HI hostels also must meet certain guidelines set out by HI.
HI has as their logo the tree and hut symbol that was originally symbolic of the International Youth Hostel Federation, who are now one of their members. HI claims something on the order of 5,000 hostels as being part of their affiliation.
One of the nicest things about HI affiliates is that they provide you with a clean towel to use.