The Google Wave demo contained a variety of "gadgets" - small little apps written in Javascript that interacts with Wave.
It's rare I see javascripts that makes me go wow!.
I've been experimenting with the canvas element myself, and there are some cool demos of it, but I loved this both because it looks good and because the resulting code is so simple..
...I've always loathed captcha's, and they've gotten progressively worse. But I've noticed some blogs seem to do reasonably well with just a single static word, presumably because most spammers won't customize things for a single blog unless you get a lot of traffic.
I also have a soft spot for an ...