Learn Arabic Language Wikibook
Summary
Wikibooks is a great idea. Like Wikipedia, Wikibooks consists of content that is public domain. That means that anything there is reusable in any fashion, such as for teaching.
The Arabic Wikibook has a nice amount of information. You can start off learning the Arabic alphabet, move on to phrases and vocabulary, start to make sentences, learn about gerunds, nominative pronouns, verbs, conjugation and more.