I have heard several good reviews of using Anki for vocabulary flashcards.
It is free online or on a PC ($20 to download an app for the iphone).
I downloaded a (free) deck of the most common Spanish words with pictures and sound clips, and Anki prompts a daily selection that I can burn through in about 10 minutes. It uses a spaced repetition algorithm, that estimates when you are likely to forget a new word, and puts it up for review that day, so your effort is more efficiently used.
It seems to be pretty helpful/effective for the minimal effort I put in.
I tried it for Chinese characters, but I think that you really need to deliberately learn them offline, before trying to review them on a flashcard system.
Thank you! I will check this out :)