iKnow grew out of a project at Cerego, a company here in Japan that made English language learning software. They moved the content online, made it free, and “socialized” it, made it have more Web 2.0 features.
After that they expanded the content to include more than English, to other languages, and then beyond langauges. All with a simple interface and a few great tools for language learners. The best part, however, is the interaction between learners. Watch the introductory video to get a quick overview. My students like this site as well.