My early favorite had to be Dan Meyer's blog, which I got into after a friend sent me his TED talk. I also found myself really intrigued by Bret Victor's Kill Math project. More recently, I've been especially taken with Audrey Watters' Hack Education, Tressie McMillan Cottom's tressiemc blog, Ben Orlin's Math with Bad Drawings, Sam Shah's Continuous Everywhere...[^samshah], and José Vilson.
[^samshah]: Come on, just look at these final projects from his multivariable calculus class. How could you not be jealous of results like that?