Make a Breakout clone with HTML5's <canvas> element
A demonstration and explanation of how to use Bloom filters
Two five minute, hopefully entertaining talks: The Malthusian Tower and Cognitive Bias and Cholera
Generate a random but reasonable NCAA bracket for your office pool
Viewji is a clone of most of NodeBox in pure javascript.
Tree drawing algorithms actually have an interesting, long history, and are fun to implement.
A visual demonstration of how a few permutation algorithms work
Hacking python's with statement for fun and profit
Two tutorials on processing images with javascript, The Histogram and Converting to Monochrome
Playing to win for the long haul instead of the short term