For many the phrase "teaching math online" evokes a vision of teaching and learning that is not based in physical classrooms. Perhaps teachers and students are even interacting asynchronously. In math classrooms in the United States, the increasing availability of devices (e.g. laptops, Chromebooks™, smartphones, and tablets) and networks allows students to access the Internet quickly and reliably. Imagining the possibilities for classrooms is an important responsibility of curriculum developers, district and school-level curriculum supervisors, and classroom teachers. The authors of this article are on the teaching faculty at Desmos®, which offers a free, online graphing calculator that runs in the window of any modern web browser. In recent years, they have been extending this technology--and merging it with their pedagogical vision--by developing a suite of online classroom activities for use in secondary classrooms, with a goal of helping teachers and students maximize mathematics learning with digital tools. The authors currently have six dedicated activities (mostly for algebra) and two tools--Polygraph and Activity Builder--that teachers can configure to meet their curricular needs in a variety of topic areas. All of this is free to teachers for individual classroom use at teacher.desmos.com. The authors begin this article by describing their vision through the principles their team has articulated in their curriculum development work. They then describe two activities they have developed that make novel use of classroom-based Internet access, including examples of the kinds of discourse and learning that these activities elicit.