Data Sources
Data Gov – www.data.gov
The World Bank Data Mine - http://data.worldbank.org/
AA Data - http://www.a2gov.org/data/Pages/default.aspx
NYC DataMine - http://www.nyc.gov/html/datamine/html/home/home.shtml
CKAN (Comprehensive Knowledge Archive Network – http://ckan.net/
Infochimps, dataset aggregator – http://www.infochimps.com/
OpenStreetMap - http://www.openstreetmap.org/
LittleSis (relationships between politicians and business leaders) - http://littlesis.org/
and as always, google is your friend.
APIs
flickr (images) - http://www.flickr.com/services/api/
last.fm (album art, songs, etc) - http://www.last.fm/api
twilio (phone calls and sms) - http://www.twilio.com/
New York Times - http://developer.nytimes.com/
twitter - https://dev.twitter.com/
youtube - http://code.google.com/apis/youtube/overview.html