Amy, check out Sanitas at http://www.sanitas.es/

I used this insurance company in addition to the regular seguridad social while I lived there. It works very well, and it even covers you when you leave the country, something you don't see too often.I used it to cover me when I moved back to the States.

However, if your job provides you with the normal Spanish public health coverage through seguridad social, you may not want more than that. It's really not bad. It has improved a lot since years ago. As a matter of fact, if anything really serious happens to you medically while you're in Spain, you are often better off going to a seguridad social hospital than to a private one.

Anyway, check out Sanitas's website - it's very thorough,includes descriptions of plans and prices, and even has an English version. I found them very easy to work with.