I'd really like a Khan Academy-like site, maybe with explanations from different textbooks for each concept. Of course then you'd need a good set of diverse problems or a way to generate such problems.
It's a shame that Khan has gotten less diverse in subjects. They used to have medicine and bio stuff among others, but now it's mostly maths. The maths lectures and exercises are still great, but even those seem limited (still no discrete maths last time I looked).