JSONP-capable API's

Just some popular sites that provide JSONP API: 

Twitter http://api.twitter.com/1/statuses/user_timeline.json?screen_name=[username] 

GitHub http://github.com/api/v2/json/repos/show/[username]?sortBy=pushed_at 

TwitPic http://api.twitpic.com/2/users/show.jsonp?username=[username]

Flickr http://api.flickr.com/services/rest?method=flickr.people.getPublicPhotos&format=json&api_key=somekey&user_id=[userId]@N00&per_page=15

Do you know more, dear reader?

Testing your application for slow connections

When there's a need to test bandwidth limitations locally you can use Sloppy:

http://www.dallaway.com/sloppy/

It runs as a local proxy, allowing you to slow down the connection speed as needed.