I have a paid subscription to The Noun Project because I like the deep and broad selection of black-and-white icons that can be used in all kinds of documents.  The licence model is worry-free: I pay for a subscription and can use any icon I download wherever I want, without the need for attribution (even though I often do anyway). I first found out about this fantastic resource when I visited Olin College, near Boston.

From the Twitter post by Marko Denic here are some more:

• undraw.co
• drawkit.io
• icons8.com
• iradesign.io
• interfacer.xyz
• blush.design
• freepik.com
• heropatterns.com (backgrounds)

James Andrew Smith is an associate professor in Electrical Engineering and Computer Science Department in York University's Lassonde School.  He lived in Strasbourg, France and taught at the INSA Strasbourg and Hochschule Karlsruhe while on sabbatical in 2018-19 with his wife and kids.  Some of his other blog posts discuss the family's sabbatical year, from both personal and professional perspectives.