Duolingo is impressing me quite a bit... I was invited in to the beta this weekend, and have been working on my Spanish. The premise is pretty neat: you mix various language exercises with translating stuff from the web, and they use the crowdsourced translations to cover their operating costs so you learn for free. The implementation is also very good: I haven't found it buggy, and it's engaging, which is really important for language learning, especially when it's a hobby. Currently, they only offer English, Spanish, and German, but more languages (supposedly including Klingon!) are on the way. I don't think it's sufficient to become fluent, but I think it can help one reach basic competency in a practical and enjoyable way.