Geogebra is an open-source product that can be used as a desktop application or online via a web browser. It is similar in functionality to Geometer's Sketchpad, but it is free and open-source. There is a bit more of a learning curve than you would find with Desmos. If you already know how to use Geometer's Sketchpad, it is an easy adjustment. I prefer the desktop application (possibly because it is the first one I used). There are slight differences between the desktop and the web versions. Software products in this suite are proliferating; I have used the "classic" version. My Geogebra documents below work best with a downloaded and installed desktop version of Geogebra (rather than the web version).
After introducing the unit circle definition of the sine function, I like to have students use the following "Unit Circle Exploration 2" to find y coordinates for angles in each quadrant. Having students add their (angle, y coordinate) data to a shared Google Sheet which has been set up to graph their points creates a slow reveal of the sine curve. In a class of 16 students, you get 64 points. Students who make mistakes can usually spot the fact that their points don't seem to be in the right place. It's a fun class activity to generate a sine curve. Obviously, this could also be done for cosine and tangent.
Linear Regression Exploration
This Geogebra document allows student to select 5 data points. Guess the regression line. Check out squares built upon the residuals. Try to move the line to "minimize the sum of squares." Finally, the actual regression line can be shown.