http://webstervanrobot.com/ (only tested on Firefox)

The program implements a small programming language on top of
Javascript that allows a robot to move around a grid of streets and
avenues.

It got me acquainted with the following concepts:

  creating objects
  manipulating DOM
  parsing strings
  implementing data structures
  responding to events
  setting up callbacks and responding to them
  variable scoping (this, var, etc.)
  cross-browser pain (which I have punted on so far)

If you're learning Javascript, this might be a fun program to hack on,
because it's all self-contained and not tied to any domain, and I'm
hoping to make it a useful program.  It really is a "Javascript
program," as opposed to code that lives within a larger system, for
better or worse.

Some challenges:

  1) Make it work on a browser other than Firefox.
  2) Change the world to be rendered on a canvas instead of tables.
  3) Add scrolling to the world.
  4) Add new capabilities for the robot.
  5) Add new syntax to the programming language.
  6) Simply play around with the code, try to restructure it to your
liking.
  7) Get the program to have a mode where you can click on it to see
the internal data structures that define the world.

Yes, I'm subversively asking for help with my program, but the program
was written with the intention of helping people learn Javascript (and
programming in general).  It's free software.

Cheers,