Language Learning Tips on Using Free Wikimedia Resources
This article points out six useful ways to use Wikimedia content from Wikipedia, Wikibooks, Wikiversity, Wikinews, Wiktionary and Wikiquotes for learning languages free.
What's highlighted here is also great for teachers because it is licensed via Creative Commons and available to use for free.
Spanish is used as an example here, but the same ideas are applicable to all languages for which content is available.
1) Know General Info about Your Target Language
Find out general information about a language by viewing that language's Wikipedia entry. For example, here's the Spanish language entry on Wikipedia:
http://en.wikipedia.org/wiki/Spanish_language
It contains loads of general information, as well as specifics about grammar, alphabet, pronunciation, etc.
2) Use Wikibooks and Wikiversity Courses to Get Started