Khan Academy is the best online tools for learning math and science that I have been able to find. It is an incredibly well thought out website, both in structure and content. In addition to having all the practice problems you could hope for, Khan Academy also features a massive range of instructional videos to help explain concepts in math, science, physics, electrical engineering, history, politics, etc.
The Khan Academy Youtube channel is also well worth checking out. It features all the videos that are embedded in the primary Khan Academy site, and thousands more that are not.
Attached below are some example videos from the site.