You can use language courses developed for U.S. diplomats for free. The courses, which usually feature PDF texts and MP3 audio—plus tests—were created before 1989 and are now in the public domain. Currently, you can't download anything, because the site's servers got swamped after a mention on Lifehacker. But I thought y'all might want to bookmark for future use, once things get up and running again.
Maggie Koerth-Baker is the science editor at BoingBoing.net. She writes a monthly column for The New York Times Magazine and is the author of Before the Lights Go Out, a book about electricity, infrastructure, and the future of energy. You can find Maggie on Twitter and Facebook.