HeyMath
The HeyMath platform includes an online repository of questions,
indexed by concept and grade, so teachers can save time in devising
homework and tests. Because HeyMath material is accompanied by
animated lessons that students can do on their own online, it
provides for a lot of self-learning. Indeed, HeyMath, which has been
adopted by 35 of Singapore's 165 schools, also provides an online
tutor, based in India, to answer questions from students stuck on
homework.
[via NYT]