My Take
I wouldn't use this website to teach a topic, but I would use it to practice a concept. It has symbols all in one place and they are free. You could also pay and create more favorite folders, but I do not mind doing a little searching. I wouldn't mind if they created some colorful ones as well, but symbols usually are black and white.
How I Use It
This website has thousands of icons that cover all the nouns you can think of! This can be used in many different types of classrooms in many different ways.
Special Education - need symbols for a schedule or for a behavior checklist? I would use these symbols in a class with students that have communication difficulties. Why buy all those symbols, clipart, and icons for books or charts (or PECS), when this is free and could serve the same purpose?
ESL - Think of how these symbols can be used to help students of a different language learn what the words mean. It would help them see the word and associate it with its meaning. Eventually, I would take away the pictures and want my students to understand the word in isolation. This may also help teachers communicate with parents that speak a different language.
Now for things you can do in the classroom - think infographics, charts, schedules, projects (brochures, creating a company logo) diagram, or with multimedia. You can use these symbols for teaching about nouns and have students create a page of nouns. Maybe you can make your own early childhood book that has sentences with symbols or have students create one. You can have students create dictionaries with symbols. This list could go on and on. Be creative!