What APIs/Web Services do you use?

I am starting the final chapter of my book at the moment (yay!) and it is “Using Third Party JavaScript”. In the chapter I will introduce some libraries and services to use and give examples how to do that (quick Google Map, Flickr stuff, using the REST APIs of Yahoo) and I wondered what other services you like using and why? I’d be especially interested in services that provide JSON objects or even code.

Thanks for your input!