Booking Rooms and Apartments in Spain with AirbnbAirbnb is the new arrival on the accommodation booking scene. It allows you to book private rooms at local individuals' own homes through their secure website. As an example heck out some of the excellent deals you can get in Barcelona: Apartments and Rooms in Barcelona on Airbnb
Or browse all Apartments and Rooms on Airbnb
Booking Hotels in Spain with Venere or KayakVenere is an easy to use website with choices between cities, countryside and coast as well as handy preference settings, allowing you to look for apartments, residences or any star hotel you like.
Venere has my favorite interface of any hotel booking site. Plus, a unique feature of their service is that you pay nothing when you reserve, so you are free to cancel later.
Kayak is an aggregator of hotel booking websites. The hotels they search range from the lower priced places to the most expensive hotels in the country.
Booking Hostels & Hostales in SpainHostelworld is great for the very cheapest accommodation. They mainly have backpackers hostels (also known as youth hostels), in which you share a room with a number of other people, as well as a few hostales, the Spanish word for guest house.
Breakfast is usually included at the backpackers hostels but not at the hostales.