Here are a short list of sites to visit when searching for budget accommodation while travelling.
Hostelbookers – no booking fees
Hostelworld – more established, so has more reviews and usually more inventory/choice of accommodation, however does charge a booking fee. Note that not all taxes for hostels are included in the price (particularly for Canada and US locations)
Hospitality Club – free, stay with locals!
couch surfing – free, stay with locals, popular site!
Airbnb – new, people listing places privately owned for you to stay – variable pricing.
For a $10 membership (often Hostelworld has offers where you get free booking fees), Travel Bunny finds Hostelworld the most useful in planning a long overseas trip. However, Travel Bunny usually uses a mix of the above sites when travelling.