Agencia Tributaria (English)
The official web site of the Spanish tax authorities. You will find many answers here, but most of the site is in Spanish (and the other official languages of Spain: Catalan, Gallego, Basque and Valencian). The exceptions to this are articles to help file the non-resident income tax (English, French, German), and a few articles on Spanish customs and trade taxes.
On the lower right, you can find all the tax forms and instructions (Modelos y formularios), as well as the filing due dates (Calendario de contribuyente).
The site has a list of countries that have tax treaties with Spain in order to avoid double taxation, with links to each agreement.
Mercantile Registries (Spanish)
The official Spanish registry of companies. All the information is online: go to their Electronic Registry (English, French, German), pay with your credit card, and you can receive information about any company in Spain. See our company search page for more information.
On the main page are the addresses of the provincial offices and the Central Mercantile Registry in Madrid. You will also find information about the Property Registry.
Spanish Notaries (Spanish)
The official site of the Spanish notaries. This site includes good articles in English about buying or selling a property, forming a company, granting power-of-attorney, and an especially good article about wills. The site allows you to search for a notary by name or by city or zip code. (When choosing a notary, note that a notary does not need to be local. Regardless of what you're signing, you can choose any notary in Spain.)
SEPE (Servicio Publico del Empleo Estatal, the Ministerio de Empleo's site) (Spanish)
SEPE replaces INEM as of 2012. All the information about contracts, labour regulations, public money and support, benefits, and vacation rules are in this site. You can download forms to hire an individual in Spain, and if you have the right electronic certificates, you can manage the hiring fully online. See their Guide to Working in Spain (in English).
Social Security (Spanish)
Spain's national health insurance provides full coverage: universal medical coverage, as well as for disability and for children who lose a parent. This site is recommended reading before hiring workers in Spain.
The vida laboral is a report from the social security office that gives the history of your contributions to social security in Spain. To get a vida laboral, call the social security office: they will send it to your address on record. To change your address, you need to go to the social security office in person.
ICEX (Instituto Español de Comercio Exterior) (Spanish)
Spanish Institute for Foreign Trade. This agency serves Spanish companies to promote their exports and facilitate their international expansion. You can find data and statistics on the site, as well as a listing of Spain\'s commercial offices around the world. For businesses, we recommend that you first get in touch with a commercial office before contacting the Embassy or Consulate. The commercial office can facilitate a list of companies in your field if your interest is to buy Spanish products. If you only want to sell in Spain, then it will be harder to get information. The ICEX site also has some reports in English written by a private firm: not always updated and not so practical, but a lot of information.
Ministerio del Interior (Spanish)
This is the Ministry that deals with DNI cards, passports, and immigration issues.
For immigration, see this link to Extranjería.
There's an article called Regulations governing EU citizens in Spain (English) that answers questions such as "When do I have the right to live in Spain permanently?" and "Can my non-EU family members live for more than 3 months in Spain?". (Note: this information may well be out-of-date, since I can no longer find it online.)
Ministerio de Industria, Energía, y Turismo (English, Spanish)
This Ministry has a site in English called Invest In Spain. See especially their search tool for grants and incentives that are available, and their article on corporate taxes.
Ministries of Spain
A list of links to other ministries of Spain.
Fàbrica de Moneda y Timbre (English, Spanish)
This office is now in charge of issuing digital certificates.
Law firms and related services:
USA: Tuan Olona LLP