I've made this topic a sticky as I'm sure a lot of us will need to get a Schengen Visa for our wives at some point. The process for applying for a visa is fairly uniform across all EU countries. The Schenhen Visa covers the following: Austria, Belgium, Denmark, Finland, France, Germany, Greece and Greek Islands, Italy, Luxembourg, Holland, Norway, Portugal, Spain (to include Canaries and Balerics islands) and Sweden.

With Spain, you need to call a Premium Rate number 0906 782 7674 (£1 a minute), and they will send an application form. The form needs to be filled in and returned, and an appointment will sent by post within 7 days. There's a 3 week waiting list at the moment for interviews. The interviews are held at London or Manchester.
You are required to bring:
Passport,
Passport sized picture,
Proof of address in UK (bank statement, tax bill, etc)
Proof of sufficent means of support (again bank statement showing regular income),
Proof of Employment (letter from Employer)
Health Insurance (get the EHIC card EHIC - that'll do,
Purpose of Visit with evidence and
Proof of Relationship - this is the marraige certificate and the original passport of spouse, NOT copy..

As Pete says, the visa is free, but remember to bring a pre-paid envelope, so they can return the passport and visa