The coolest things about canvas is that it can be used to build simple but useful client-side graphics applications that run inside web browsers. These applications can be quite sophisticated, with cross-browser compatibility and a complete point-and-click UI component.
On my website, I have some rather simple examples of what canvas can do. These include applications for generating fractals, cellular automata and for a 1D, two-phase lattice Boltzmann method. I plan to spend a lot of time with canvas in the future.