It's been a (long) while since I came across anything really new or exciting in the world of online language learning, but Duolingo looks like it could be both. And it's completely free. To find out more, read this brilliant review by Adam Costa. Also check out the fascinating TED talk by Duolingo creator Luis von Ahn.
At the moment, English is only available for Spanish speakers. Hopefully, more language options will be added soon (English for French speakers, please!). I signed up for the German course (for English speakers) and spent about twenty minutes working my way through the beginner's level (I haven't done any German for over 30 years). I found it quite motivating, though more advanced learners might find it frustrating not being able to choose their point of entry without doing a test. If you are interested in learning a language online, you should also have a look at busuu.com, which has more language options, but is less motivating IMHO. And make sure you check out the Top ELL Sites in the menu on the left.
• duoblog (Official Duolingo blog)
• Duolingo Teaches Languages Free While Translating the Web (Newsy video)
• Duolingo Review (PCMag)
• Duolingo: Learn a Language While Translating the Web (PCMag)