This could be useful for people learning a new language. There are already plugins for getting translations of words/phrases, but if this service also stores the words I've looked up, I can go back and repeat.

Maybe I can even be quizzed on my stored words?

With device syncing, I can read and add words during the day, and then I can get quizzed on my Android phone first thing in the morning before leaving bed.

As I like learning new languages myself, I'm thinking this could be quite useful. I realize this is probably drifting a bit from your idea though.. Just thinking out aloud.