The Shelter Selector
Except for tenting, hostels are far and away among the cheapest places to crash. A clean bed and much-needed shower typically go for a fraction of the price of a hotel room. You may give up some of the luxuries a hotel offers, but hostels are also unbeatable for meeting fellow travelers and exchanging notes on the local scene.
The Hostelworld app offers a very comprehensive list of hostels and reviews, and is available for both Apple and Android devices. It also lets you look for hostels before entering any booking information.
A newer entry in the mobile market is Hostel Hero. With its latest version, it removed booking fees and added reviews. Hostel Hero licensed Hostelworld's database, offering location-based searching for nearby services, downloading for offline browsing and hostel reviews. Unfortunately, this app requires that you enter booking information before you get to see any hostels. While not an obstacle for many users, it'll be an inconvenience to others.