Desmos is a free graphing calculator that you can use in your web browser on a Chromebook or any other computer using the Chrome browser. It is also available for mobile phones. Desmos has a robust website full of activities and lessons to use in your classroom.
Graph.tk is a free online graphing utility that allows you to plot multiple functions through its dynamically re-sizing grid. To graph an equation on Graph.tk just click the "+" symbol to enter a new equation. One thing that isn't clear the first time you use Graph.tk is that you need to delete the existing default equations before you start.
GeoGebra Classic is the browser-based version of the popular GeoGebra software. For more than ten years teachers have used GeoGebra to teach graphing functions, probability, data analysis, and much more.
Math Vocabulary Cards is a free Chrome app designed for elementary school students. The app offers exactly what its name implies, a series of flashcards of mathematics vocabulary terms. Each card contains a term, a diagram, and a definition. By default the term is hidden and students have to guess the term based on the definition and diagram. Students can also use the cards with the definitions hidden and the terms revealed.
Geoboard is a free app on which students stretch virtual rubber bands over pegboards to create lines and shapes to learn about perimeter, area, and angles. The app is available as a Chrome app. It can also be used directly in any modern web browser. The browser-based version can be found here.