I read somewhere in the distant past that posting questions and getting answers to exercises from a textbook in a public forum like this one reduces its value in courses.
Wikipedia has a nice collection of articles on mathematics.
ProofWiki has lots of proofs, although I can't tell what license it uses for its content.
Is there an open source / permissively licensed collections of exercises, especially ones that target a variety of mathematical fields/disciplines?
There are the lists of problems from the American Mathematics Competition and such. However, if you would want a dynamical engine that continually provides you problems that adapt to your skill, then Art of Problem Solving's Alcumus is a good resource to use, as they host $13,000+$ problems with solutions.
If copyright is hyper-massive concern, on top of not using mainstream textbook exercises, then I (and my co-author Anon) are willing to give you (anyone reading this) explicit permission to use one of our universities problem-competition books here we published a while ago, as long as proper citation and attribution is used and also it's non-commercially used and strictly educational. It targets a large stream of math topics, and was specifically designed to encompass students of all math levels (cyan = beginner, green = at least has taken Calc I or II, and magenta = advanced).
You can also get a bunch of free math problems from sites that provide Open-Access textbooks and other Open Educational Resources for Mathematics, as they are not commonly used in class rooms, and mostly supplemental. Copyright will be subjective to with material you use, so do look closely, but the bottom line for most is proper citation and attribution.
