I have been using flashcards for quite sometime now and these are undoubtedly a great asset to anyone trying to learn a new language. While I admit I can't regularly peruse the daily email-digest I get, I try to do a batch-full-of-'em every now & then and this also helps. The email is formatted nicely enough to be rendered on a smartphone such that one can revise/learn while on a move.
The best part is this is free service offering a lot of flexibility. There are paid options too, which I am sure offer much more and would likely be worth the price, but I think the free service suffices for me as of now. And I don't recall seeing any stupid advertisements or getting any additional spam (well, once or twice there was a genuine-looking-announcement seeking partners for playing badminton, a sport I absolutely love myself ;-) so I was rather sorry I couldn't reply to them as unfortunately, I live a few thousand Kms away from their location :-D).
Basically, I am quite happy with the flashcards, would recommend them to anyone, and in the future, may subscribe to one of the paid services as well.