Virtual Mathematician Grapher

Virtual Mathematician Grapher is a free graphing app that can graph any closed form function. You can customize the colors of the graphs, you can have multiple graphs on one screen, and you can have complex-valued inputs (and get complex-valued outputs) with your graphs. This app is totally free now, so download it as use it as you please, forever, for free.

Recently updated for Windows 8.1. You can now run it in any snap position at all (even the tiny 320 pixel position), and you can take a screenshot of your graph using the share charm. I also made it so that if the OS automatically terminates the app, your previous graphs are restored next time you start the app up.

Now go and make pretty graphs!