Here are a few of the online services I've been playing with so far.
Live Mocha is a language instruction site emphasizing social networking. They support a wide variety of languages. Online lessons are available. They look a bit like a poor man's Rosetta Stone. They're free, but I would not recommend relying on them to learn a language. More advanced versions of the training software, including tutorial sessions, are available for a subscription fee. The more innovative aspect of the system is in the social networking end. You can set up chat sessions in your target language with other students or with native speakers. This is helpful, though as I said last time, I'm not sure my chat skills are all that good even in English.
A similar site I haven't tried is the Spanish-based company Busuu, focusing mainly on West European languages.
Zon is an intriguing site to teach Chinese, presented as a multiplayer online game. You create an avatar for your character, who navigates around different scenes. You interact with different objects to get lessons, exercises, and so on. This shows some promise, though I haven't gone very far with it yet.
Italki is another social networking site for language learning. Based in Shanghai, it provides no language learning resources of its own, at least along the lines of what LiveMocha has, but acts as a network and marketplace for students and teachers. Apart from that, it feels much like any other social networking site. The ready availability of specialized groups and forums is something that I found lacking in LiveMocha. The market provides links to language teachers, most online.
This last feature is something I find intriguing as I study the history of language teaching. It seems as though most language teaching sites are now emphasizing providing online tutors and instructors. Even Rosetta Stone recently announced the creation of a new subscription service which includes, among many other features, access to "Studio Coaches" who are native speakers in the target language. On the one hand, this development seems like a natural extension of the increased communications available, and a useful way to increase practice in the language. On the other hand, I'm left to wonder how the supply of competent language tutors will be able to keep up with a growing number of students online.