Lithuanian

From UniLang Wiki

Lithuanian is one of the two living Baltic languages, the other one being Latvian. Lithuanian is spoken by four million people today and it's often considered the most archaic language in the Indo-European family of languages.

Lithuanian Grammar
Guide to lithuanian language
Early Indo-European Languages Online A great site that has lessons in several Classical Indo-European languages, including Latin, Classical and New Testament Greek, Old Norse, Old Church Slavonic, Old Persian, Avestan, Lithuanian, and Hittite. And the site keeps growing! It's run through the University of Texas.