KhanAcademy has a lot of CCSS-aligned exercises, many of them having rich js interactives. The coverage is also very good, probably better than anything commerc

KhanAcademy has a lot of CCSS-aligned exercises, many of them having rich js interactives. The coverage is also very good, probably better than anything commercial out there. There is also an initiative to fund CC BY licensed educational content that school districts can use without getting milked by textbook publishers http://k12oercollaborative.org/ http://k12oercollaborative.org/