If you dont use this in some form in your classroom you should start thinking about a way. This site is great. I use it in a bunch of ways;
1. Graphing Calculator
When a student is unsure of something, how it looks or what happens when I change ______ we pull up desmos and look. When we are introducing transformations, its so easy to get an equation with variables and use the sliders. For projects where students have to create a graph and then come in with it printed. Desmos is there for us.
Any of their pre-made activities are great, or just create your own. Easy entry but a lot can be done with it;
Card sort, Marble slide, Graph draw, Create your matching graph, Hidden folders...
At the start of class have students create a function that shows a property, or display a property that I want them to then show me another function that has the same. Its an easy way to see if they understand or don't. Plus in small groups they can work together or separate and then compare what they created.
Echo Smart Pen
I haven't used this yet, but my department got one over the summer and I want to pilot it. The idea is to have students take it home and do a homework problem while using this pen. The pen records their voice and their scribe to a file that can be then viewed.
Its a way to get into a student's head, and hear their thinking out loud.
I use this every day in my classroom. This site is a quizlet type site, a kahootz type site. It is a site where you can pre-make a quiz, and have students do it with instant feed back while having teachers see how many correct and all that. Students can access this on any device. Computer, tablet, smartphone, ipad... all of the above, just need an internet connection.
What I use it most for is the exit ticket. Its pre-made and leaves a question at the end for you to make up for your kids. So you could ask, what did you think of? OR what would you change about? OR how would you? and at the end there is a free response question for the students to do. I usually ask what students would change or improve about the lesson. Probably 60% of their feed back is "I liked it all" or "Dont change a thing" But every once and a while you get that diamond in the rough, which makes you think... oh yeah, I could do that!