We've probably used this site and app the most when it comes to finding accommodation. It seems like most if not all hostels, homestays and apartments available are listed on this site. You can filter by price and reviews which have been the most important for us. The nice thing with booking.com is you can reserve an accommodation without your credit card. You have the option to pay later (at check-in) and you can cancel free of charge by an assigned date. Also, if you become a genius member like us (occurs after 10 bookings) you receive an additional 10% off some accommodations. Other perks of being a genius member include an early check-in, late check-out and a welcome drink. :)
This is another favourite and we've used it almost as much as booking.com. I don't like that they charge your credit card the moment you book and a lot of rentals come with bogus cleaning fees. Additionally, we have had a few hosts cancel our reservation very close to our arrival date. It is still a worthy consideration if your looking to either rent a shared room, private room or an entire home.
This one is similar to Booking.com and is considered their sister site. The Priceline Group owns both booking.com and agoda.com. We found Agoda to be priced slightly better in Asia (on a few occasions) because the Asian market seems to be their primary focus. Additionally, unlike booking.com, agoda will charge your credit card either immediately or by an assigned date. You don't have the option to pay cash on arrival.
We don't find the best deals here. We use expedia for travel reward points on my visa when their prices are comparable to other competitors.
Another good site that's useful for comparing accommodation prices. It groups everything together from Booking.com, Air BNB, Roomorama, etc.
I just have to mention Couchsurfing here because as much as we wanted to love it and use it, we haven't had any luck. People either don't answer or their place is not available OR they don't seem to like to host couples. If you land a place to stay with a host, it's free! There also plenty of house sitting sites you can try for free stays, but we found them also to return with very few responses and/or want people to stay for very long stays.
World Wide Opportunities on Organic Farms is what "WWOOF" stands for. Its a program set-up where you get to live and eat for free in exchange for approximately 4-6 hours work on an organic farm.
Its help exchange for food and accommodations. It differs from WWOOF as it offers you to be placed not only in farms but in big cities. Exchanges in hostels are common. This program is well suited for city seekers, not just rural lovers.