In combination with some JS libs you can use it as a graphic design editor, like for t-shirts or those silicon phone cases.

We're using it with video to capture snapshots and as a base for face recognition on PukkaTeam.com

You can also use it to power your web apps. Flipboard mobile website uses canvas to get 60FPS animations in mobile. You can read about it in detail in their blog.

I made spirite editor with it. Usprited

As a maintainer of http://konvajs.github.io/ I am using <canvas> a lot. Some visualizations, graphic applications (e.g. floor plan builder).