Quizlet is an online learning tool that allows you create and study flashcard sets. Using the flashcard sets, students can learn in six different study modes: Flashcards (flip through the material), Scatter (match words to their definitions), Speller (type what you hear), Space Race (type words before their definitions vanish off screen), Learn (track your progress over time) and Test (graded quiz).
You can create flashcard sets in a number of foreign languages and can also have words be matched with Creative Commons images. Quizlet is free, but paid accounts are also available if you want access to features like the ability to upload your own images, to remove ads for your students, and to create an unlimited amount of classes. An iOS (no Android, sorry) app is also available.
Check it out here: http://quizlet.com