Intense German – learn German words fast!
My girlfriend recently told me about a quick way to learn a lot of vocabulary called the Iversen’s Method. In this method you get a list of seven words to learn, then you keep shuffling them and go through them again until you remember them all. Then you get another list of seven words. After a while, you get your old words to review again. To my knowledge, no other app uses it, so if you want to learn this way on the go, I guess you better get an iPhone. 
She compiled a list of 2700 words in the following categories: Basics, Slang, Professions, Languages, Countries & continents, Numbers, Question words, Words for time, Weather, Family, Emotions, Clothing, Colors, Describing people, Town & outside, Traveling, Body Parts, Eating, Foods, Parts of a house, Furniture, Animals, German 201 words, German 250 words, German 301 words and Advanced words.
Try it for yourself for only $5!
Anki does something similar, I use it every day:
http://ankisrs.net/