I've been using Quizlet for a while while learning French. It certainly has come in handy for vocabulary, but as with a lot of sites based around user-generated content it's really hard to find the good content that I want.

For example, there are thousands of French flash card sets, but not all of them have English as the first language (and some of them are French and English but English is the second language). I've found that really the best way is to stab in the dark until you find a user who has created the best content.

Oh, and while you're here, it would be great to have some nice smartphone formatting or an app for doing revision on the move.

Looking forward to the audio features!