Shameless plug: two years ago I created http://nabbber.com to help me learn English. Basically, it's like Twitter, but instead of tweets you post words that you've recently learned. You can follow other people and see their new words on your home page (just like tweets).

I'm in Bali now and I'm using Nabbber to learn Indonesian. Whenever I see an unknown word (in a shop, at a gas station, etc) I save it and then revise from time to time.

Here's how my profile page looks like: http://nabbber.com/AlexanderZaytsev/words/id:en

I find this method (just got out in the wild and write down unknown words) the most efficient one.