Over 50 years ago, the Unites States Department of State Foreign Service Institute (FSI) created a program for teaching Spanish. It was used mainly with government employees, including diplomats, CIA agents, and others. By all accounts, the program was very effective. There was no effort to make it fun.
That program was in the public domain, which means that anyone can repackage it and resell it. A number of companies have done this with the old FSI Spanish program, sometimes selling their version on its own, and more recently including it as an add-on to newer products.
But it is available online in download form, for free. That’s the good news. The bad news is that it’s quite out of date and the sound files are poor quality. I listened to one and couldn’t bear it. More good news: there are updates; see below.
The website I found seems to be a total labor of love, which has available for free not just the Spanish course, but also classes from Amharic to Yoruba, with several languages I’ve never heard of. As it is a volunteer effort, not every bit of every language is there, and it seems that not all the Spanish has been done yet.
Here is the Spanish download page. And here is a forum page about Spanish on the same site.
Updates to FSI Spanish
Don Casteel, an American career diplomat in Latin American countries, learned his Spanish with the old FSI course, and he took on the job of doing a complete revision of the course. This was published as Platiquemos, and you can read about it at Don’s website, http://www.platiquemos-letstalk.com
I bought it from there around 2005 and had some correspondence with Don at the time. Since then, he has passed away, but the website is fascinating. I don’t know if anyone is filling orders from there, but you can also get it at MultiLingual Books.
Here is my review of Platiquemos written several years ago: http://www.mexico-with-heart.com/otherbooks/platiquemosespanol.html
Platiquemos is no longer such a favorite of mine, now that I have discovered some other ways to learn Spanish, but it is a good workhorse. I am not a fan of the old FSI course, for reasons I mention above, but describe it here because someone may want to use it.

