The Noun Project is a website that hosts icons created and uploaded by graphic designers worldwide. These icons can be used for free (with attribution) and are licensed under Creative Commons Attribution (CC BY) or Public Domain (PD).
I use these icons in many of my FCC projects and even in presentations that I’ve given. All you have to do is search for any noun, and The Noun Project will give you back a number of related icons. You can search for weather icons, pizza, even zombies.