Useful website links for studying materials

Hey wanted to collaborate on any website links that people have that they feel has helped them master a concept or prepare for an exam that is outside any published materials. Like websites that explained a concept in a different way, more practice problems ect. Please include the topic covered and the level it was pertaining to. Video links preferred but pdf’s are great too. Example: Topic: BA 2 Plus Pro Video Tutorials Test: Level 1 Link:

Oh and because I forgot to say it: I was looking for free and legal links to whatever content is out there not links of things to purchase there is plenty of advertising on the site already, we don’t need more in this thread.