Data.gov is the US government's source for government data. It is free, which means it is funded by tax payers. I find this source very interesting.
Another interesting data source is Quandl. I tested some of the Finance and Country data APIs from Quandl using Google App Inventor's API features. The data platform is well organized and well documented. I am amazed at the ease with which all this data can be accessed and consumed. A good programmer working with a good product manager can work wonders with this new oil.