Here are my favorite extensions lately:

Classroom for sharing directly to your courses.
Scholar, lookup scholarly articles while you browse the web
SMART Amp (free trial) for sharing notebook style information directly to your students in Google Classroom
Goo.gl link shortener
Readablity for a simplified reading experience on the web
Balsamiq (free trial) to mock up a website or app (this is fantastic for student projects in the prototype phase if the end product will be a webspace)
PowToons animated videos and presentations
Datum Equation Editor