So the other day I was burning time doing a little bit of Spanish practice on the free version of duolingo when I thought to myself "wouldn't it be fantastic if something freely available like this existed that comprehensively taught mathematics up to a university level".

Does something like this exist and if so is it any good? Ideally the software I imagine is totally comprehensive; offering not only tutorials on everything from the absolute fundamentals to advanced topics but also problem generators that allow people to apply the lessons, practice and test their understanding under a wide variety of possible circumstances. Does such freeware exist?

A readily available, approachable and most importantly freely available resource of this kind could be a huge boon for spreading mathematical education all over the world.