Youtube channel by the journalist Brady Haran, interviewing mathematicians about interesting and playful mathematics.
Youtube channel by the former Khan Academy employee Grant Sanderson. The mathiest mathematics youtube channel.
Series of articles in Notices of the AMS, introducing different concepts from modern mathematics.
MOOC platform, featuring (mostly) free mathematics, cs, and science courses at the undergraduate level. Includes basic quizzes and interactive assignments.
The brilliant and prolific Evan Chen summarizes undergraduate mathematics in a series of short olympiad-style chapters; with exercises.