American Institute of Mathematics Open Textbook Initiative --
note that they review the texts too and are a bit picky about what they list: https://aimath.org/textbooks/
More than just math: University of Minnesota open textbook initiative. Stats, CS, and humanities as well: https://open.umn.edu/opentextbooks/
Not a repository, but an individual free/open math text under development -- comments and feedback desired: https://www.softcover.io/read/bf34ea25/math_for_finance It starts with elementary probability and then combines probability and stats with linear algebra, multivariable calculus, and differential equations. Aimed at folks who have seen the math before but need a refresher and a viewpoint that unifies seemingly disparate topics. Note that it uses Softcover, a great way to publish technical texts to several formats at once.