Duolingo is the brainchild of reCaptcha inventor and professor Luis von Ahn. His idea was to teach language on the web while recruiting the students to translate the web. It has a number of cute features, in addition to getting students into translating real stuff at an early stage. There are social network features, and my favorite, a video game like interface where you accumulate points and level up by completing lesson and doing translations.
Leveling starts out a bit easy, I think - it only took me 400 points to reach level five, but level six required 200 more and level seven is 400 more. So far the vocabulary is fairly complimentary to Pimsleur.