Paul's Online Math Notes has notes on topics from algebra all the way through differential equations.
The Bright Side of Mathematics YouTube channel has videos on all kinds of math topics from logic to linear algebra to measure theory to manifolds.
Problems in Mathematics has tons of solved practice problems in linear algebra, group theory, ring theory, field theory and Galois theory, and module theory.
Physicist Leonard Susskind has some continuing studying lectures called The Theoretical Minimum that cover classical mechanics, quantum mechanics, special and general relativity, cosmology, and statistical mechanics that I really enjoy. The first four of these (classical mechanics, quantum mechanics, special relativity and classical fields theory, and general relativity) have also been converted into books that I enjoyed too.
Danika Van Niel has an excellent resources tab on her website!