I've been a fan of The Noun Project for a number of years. Their recent collaboration with Google has got me really excited...
What is The Noun Project?
The Noun Project is a collection of over a million curated icons, created by a global community.
Icons are free to use under a Creative Commons Licence (so you have to attribute the work to the designer).
If you purchase a NounPro licence you do not have to attribute the work and you can use as many icons as you like. This licence costs $39.99 annually but there is a discount available for education and team users.
Google Slides and Docs Integration
The Noun Project has just announced a new integration with Google Slides and Docs. They have produced add-ons which enable you to use a selection of icons for free (no attribution required) directly in Google Slides and Docs.
The video below shows you how to instal and use the Google Docs add-on. It will work the same way for Slides,