I cobbled together a little (barely) interactive crossword puzzle engine. Here's a link to the puzzle I did a couple months ago, Transportation Alternatives.
It has some rendering problems, especially on IE, and there isn't any way to save your progress or check your answers yet, but the highlighting and everything seem to work okay, so I thought I'd link to it.
The interface is GWT which is very cool and fun. And sort of mind-blowing in that it even exists at all, let alone works as well as it does.
Update: changed puzzle link.