I love the Google AJAX APIs. It took me less than an hour to make an Ajax Wiki Search ( http://chir.ag/wiki/ ) using it last year and I use it all the time. The thing that's not clearly evident when using these examples is that it's not just replacing a one-time REST/XML call with JS but a real-time ajax query which means we can call it in some pretty nifty ways.
I've used the chart API in an internal app such that the user gets a real-time feel of the data as they change the parameters without having to hit "Submit" every three seconds. Also note, the motion api is derived from the Gapminder organization they acquired in 2007.
This is really useful to everyone exploring their APIs. I just found that the visualizations can render unicoded bangla text, which I had assumed wouldn't work since it didn't work in the charts api.
edit: Bangla is only working where they're using svg. Still broken in the bitmaps. Does anyone know where to file bug reports?
I really like those boxes that he uses to structure the site. I looked in the source... Does anyone know if the ShadowContainer is available as some type of JS library?
The guy that wrote this hangs out at irc://irc.freenode.net/googleajaxapis which is also a great place to ask questions on any of the APIs :D