The DLMF, a project of the National Institute of Standards and Technology, "was initiated to perform a complete revision of Abramowitz and Stegun’s Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, published in 1964 by the National Bureau of Standards. The DLMF Project has updated and expanded the coverage for current needs."
"The Electronic Library of Mathematics contains online journals, article collections, monographs, and other electronic resources in the field of mathematics. All material is in electronic form and access is generally free."
"Khan Academy offers practice exercises, instructional videos, and a personalized learning dashboard that empower learners to study at their own pace in and outside of the classroom." Mathematics topics include calculus, linear algebra, differential equations, statistics & probability, and more.
Overseen by the North Dakota State University Department of Mathematics, the Mathematics Genealogy Project strives to "list all individuals who have received a doctorate in mathematics," as well as their advisors and the title of their dissertation.
"The University of Michigan Historical Mathematics Collection is a growing library of books selected from the University of Michigan mathematics collection that have been digitized to improve access and to preserve the content of these books. All of the books in this collection were published in the 19th or early 20th century."