http://www.half-life2.com/hl2.php?

Ran across this. Note the default movement controls are mapped to WASD ;) (S will give you a result, highlight the page to see it)

How to play: look at the address bar; substitute something in for the w a or d: http://www.half-life2.com/hl2.php?move=(put something here). Looking at page source might be helpful, so I've heard.