Tetris in JavaScript using the Canvas element, 'nuff said!
The canvas element is awesome. JavaScript is fast enough that you can run some pretty computationally intensive stuff (I've seen 3D games, a NES emulator, and much more all done with JS!). This script shouldn't push your CPU to the limit, but it does show how easy it is to create cool effects with just a small amount of code.
Keeping with the theme of yesterday's post - "a stroll down memory lane" - I thought I'd re-create the Nokia Snake game (a distant relative of Nibbles) using JavaScript and the canvas element.