American Youth Hostel Association
This is a great place to get started. They'll help you make reservations around the world.

Eurotrip.com Hostel Reviews
This is an index by country of reviews for hostels. Keep in mind that just because someone has a good or bad time at a particular hostel does not mean your experience will be the same.

Hostels.com
In addition to the bulletin board above, Hostels.com provides links to a number of budget services. A good starting point for those considering a hostel vacation.

Hostels of Europe
This site not only lists 180 hostels in Europe, it also includes classifications that give you the flavor of the places: quiet, party, family friendly, etc. Check out their discount card.

Hostelworld
Their database is good for 26 countries, including the United States. They can book reservations and provide destination information, too.