There must be a lot of webs out there that are useful to help us studying, but today I want to remark this one: Study Stack. It is said to be the most versatile flashcards in the world, anyway I have to say that I find it very useful, you can use the already made ones or design your own ones according to your interest.
And it has a very funny point to make the kids study with games, they can get a match quiz, play hangman, or play the so visually attractive unscramble, you can alostry to do the crossword, or print it and so many options with a lot of different topics. A web that keeps on growing thanks to a lot of generous teachers and students sharing their work.
You can find sites like this and a lot more in this web about Free Technology for Teachers