Thought I would give this idea a whirl. I've seen members asking for or giving useful links to others to help them learn a language, and it reminded me that we have no general 'resources' thread in here. This thread should serve as a repository for useful online resources for learning languages - the content can be anything (lessons, e-books, videos) just as long as they are free and legal. If this picks up, I can edit the OP into a 'table of contents' listing everything that is posted.
I'll provide some links to start off.
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Early Indo-European Online - A series of grammar and vocab lessons provided by the Linguistics Research Center at UT Austin in early and modern Indo-European languages. Languages: Albanian, Armenian, Baltic (Lithuanian and Latvian), Old English, Old French, Gothic, Classical Greek, New Testament Greek, Hittite, Old Iranian, Old Irish, Latin, Old Norse, Old Church Slavonic, Sanskrit, Tocharian.
FSI Language Courses - Public domain versions of language courses from the Foreign Service Institute. Languages: Arabic, Chinese, French, German, Japanese, Russian, and many more.
Wiki Books - Some full or partially completed books/lessons. Languages: Latin, Spanish, French, Mandarin Chinese, some more.
For fun: Introduction to the Babylonian Language - Some resources in Akkadian/Babylonian, if anybody wants to read Hammurabi's Code in its most authentic form... Languages: Babylonian/Akkadian