Spain is a very beautiful and innerly different country. From the very Mediterranean, almost Arabic, Andalucia to the very proudly autonomous regions (or countries according to many of their inhabitants) like Catalunya, Euskadi (Basque country) and Galicia in the North.
Spanish people are usually not very talented speaking foreign languages which makes your life much harder if you don't speak a basic Spanish.
There is a big counter culture, from the "acampadas" (occupy) movement, to many ecovillages and communities all around the countries. Especially the south, can be very hot in summer (the whole country actually), but nicely warm in winter, making it a good idea for Winter retreats. It is generally quite a good place for nomads.
Hitchhiking in Spain sometimes might be not so easy. However, the meaning of the thumb will be very easiful understood, though probably the best method will be asking people directly whenever possible, even if your Spanish is very poor.
Accommodation & Wild Camping
Wild camping and Bivouacking
Wild camping should be theoretically illegal, though practically much more than tolerated. As usual, the less you stay and less you let others notice you, the easier will be.
It is legal to wild camp for free in Spain as long as you camp when the sun sets and leave early in the morning. This is called pernoctation and the forest guards will not bother you if you explain that you know what you are doing. Remember that lighting a fire is forbid in most of the Spanish regions.
If you go to Spain during the summer months (June-August) it will be warm enough in the night to bivouac (sleep outdoors without a tent). However, it is recommended to take a thick sleeping bag because temperatures can drop to 10 celsius degrees in the north, the meseta, and hilly areas. Although it might seem very bohemian and romantic, avoid sleeping in a beach. Especially if it is populated by drunk people and guiris (Spanish despective word for stereotypical north-European tourists that come to Spain for cheap alcohol and street sex). Chances are you will get stolen by pickpockets who go to those beaches to take advantage of the wealthy, drunk guiris.
Despite the fact that many houses have been shut down lately, Spain has a very active squatting scene. It’s quite easy to find a place to crash by asking around for a casa okupada.
Hostels and Pensions
There are plenty of backpacker's hostels in big and not-so-big cities in Spain that you will find online through sites like www.hostelworld.com . A cheap hostel you can find online might cost between 10 and 20 euros depending on the season and the region (Basque Country, Catalonia and Madrid are usually more expensive than the rest of the country).
Now, here is the little secret that Spaniards know and you don't, A vast majority of cheap pensiones do not appear on the internet and you will hardly find any information on google or tourism offices. This is probably because the owner of the "pension" or hostel might be an old lady that rents a room in some kind of not very legal way. Pensiones can vary in cost and luxury, depending on the number of stars you find under the "P" sign. The best thing to do is to go to the city hall and ask for the yellow pages book Páginas amarillas, call the pensions and ask for the price. You will need to speak Spanish here, or find someone that can speak Spanish for you because most old ladies do not speak English. It is a good idea to ask the locals if they know a cheap place to stay, or even ask so to other pension owners.
Pilgrim hostels (Camino de Santiago)
Camino de Santiago is a legendary long hiking pilgrimage trail that can be started from almost anywhere in Europe. It ends in Santiago de Compostela, a beautiful city in the heart of Galicia. Obviously, this trail reaches its highest fame rate in Spain, where it is known by everyone and is even part of folklore. Nowadays, pilgrims that go to Santiago are backpackers from all around the world with varied reasons for walking (some religious, some for fun etc). There is a broad net of albergues de peregrinos (pilgrim hostels) all around Spain for the pilgrims to spend the night after a day of walking. Those are undoubtedly the cheapest accommodation in the country. The price can vary from 5 to 10 euros and even some religious centres offer it for free or the will.
In most cases, to use the pilgrim albergues you will need to prove that you are a pilgrim with a credencial, a passport with the stamps of the towns that the pilgrim has passed by in his journey. You can get your credencial in any church/city hall through which the Camino passes. Lately, people do the Camino in many different ways and directions, Some even by car. So do not be shy about asking the albergue owners for a bed or help. If you are friendly, they will be too.
Bewelcome link http://www.bewelcome.org/places/Spain/ES
Shower & swiming
Church and govermentsocial centers has showers and option to change clothes
Dumpster diving at supermarkets and openmarket works usually well. Supermarkets and bakeries throws out expired and unsold stuff after closing. Wait until people come out with trash bags. Asking also is often a good idea.
You can find palces where homeless can eat - churches and caritas
Wifi is quite well spread everywhere, Internet cafes also.
Orange Spain has 100Mb for 5EUR/day
Depending on the cities. Euro currency but not so high average income. Quite good anyway but not as continental/northern europe though. Approximately the same as most of Mediterranean Europe.