LiveMocha is a great way to learn a language, particularly for a visual learner. Generally, you would start at the most basic level. LiveMocha shows you pictures for each word. After glancing at each of the word, hearing them, and repeating them you might still feel like you don't know what they are.
That's where the review comes in. Once you go through the review you will realize you have remembered a lot of it. If you're feeling confident you can even try writing or speaking in the language and then sharing it with the community for a review. Native speakers will then review the language and give you advice.
You can also help native speakers of other languages learn English in a sort of pay-it-forward type system. Some aspects of LiveMocha cost money but there is a lot you can do for free.
LiveMocha currently offers: Arabic, Bulgarian, Catalan, Croatian, Czech Dutch, English, Esperanto, Estonian, Farsi, Finnish, French, German, Greek, Hebrew, Hindi, Hungarian, Icelandic, Indonesian, Italian, Japanese, Korean, Latvian, Lithuanian, Mandarin Chinese, Norwegian (Bokmal), Polish, Portuguese (Brazil), Portuguese (Portugal), Romanian, Russian, Serbian, Slovak, Spanish, Swedish, Turkish, Ukrainian, Urdu