In order to discover this tag (I haven't really studied HTML5 yet), I decided to make a small Pong game, for my UniPong project.
You can try it just below (just click on the "Go" button), or check the source code. It's pretty simple and it may not work perfectly, but it should run fine anyway. It should even be a good exercise to try to reproduce it by yourself.
Have fun ! (or not)