Hostelworld/Hostelbookers (click here):
With over 36,000 properties in 178 countries with over 13 million verified guest reviews and 24/7 Customer services, Hostelworld is the authority on booking hostels.
CouchSurfing (click here) is a website where you can connect with people all around the world who offer you a place to stay for FREE. Besides accommodation and connecting with people in those locations it is a great way to meet people who can sometimes tell you about their country/culture, show you around, answer questions, practice languages, etc. It also a social networking service that has a mobile app and you can find groups, activities, etc…
Airbnb is an online marketplace that connects people who want to rent out their properties s with people who are looking for accommodations in that locale. It can be a bedroom in an apartment or the whole apartment and is usually cheaper than a hotel, but giving you the comfort and convenience (like a kitchen) of a private property. It currently covers more than 81,000 cities and 191 countries worldwide.
If you are interested in volunteering or working abroad in exchange for accommodation and food see click here