I was recently looking for some publicly available data for a demo. Many times, the complicated solutions we deliver for clients are under NDA and cannot be shared as Demo material for features or to new clients so I'm always in search of free public data that has useful business questions to answer.
That is when I came across www.Data.gov This is the governments repository for all kinds of data in many different formats for public consumption.
If you need to find some interesting data this site has it all. It has everything from Airline on time data to world health data, education data, etc.. Great source for what demos or features you may need to test.