I have been using iKnow extensively to improve my Japanese for my classes. Today I found out about another site called Live Mocha that offers social language learning. They offer Mandarin Chinese, Spanish, Italian, French, English, German, Russian, Hindi, Icelandic, Portuguese, and Japanese. Its another great resource for language learners. So many great sites are coming up online, its so hard to keep up.
Update: I found another site called italki that offers social language learning but mostly focused on finding people who can help you learn a langauge versus actually giving you tools to learn. Its a nice site, very similar to Live Mocha.