Math

[A terrible taste]

No seriously, what level of math?

[Kingz V]

second for khanacademy, youtube is great as well there are a few gems.

interestingly enough there are some weird math torrents out with some interesting content.

a workbook/sheets related to your topic might be most beneficial.

my friend also uses an app that quizzes him on math in the morning with his alarm clock. don't have an app name for that though.