CANVAS for FUN

Explore black pixels of Mandelbrot set, their path is quite surprising.
Bumpmapping effect with interactive light.
Play with interactive graph.
[inspired from applet demo]
View panoramic photo along with interactive 2D radar map.
Play with time by revealing older picture with mouse.
[bug with Firefox]
Move bunch of sprites thanks to canvas, revival of famous challenge on Atari ST.
Interactive timelapse with mouse controlled non-linear time.
Play wih cellular automaton (Conway's game of life...) with *your* rules.

CANVAS for MATH & FUN

Javascript and canvas can be useful for math stuff! It can handle huge numbers (>1000 digits), is interactive, and quite fast, see it in action:
More demos: js1k.com, canvasdemos.com, wolfenstein engine on nihilogic, raytracer on nerget.com, ...
Useful specs: canvas specs, canvas cheat sheet.