As Tim Berners-Lee told at TED, there are many hidden data. What helpful free databases or APIs you realize around web? For instance:

UPC Database (personal only use)

There's lots of assets listed in the following paragraphs (and also the comments too) at Read Write Web: Where to locate open data on the internet

See also the ProgrammableWeb API Dashboard — APIs are listed by category and recognition.

world wide web.freebase.com

The IP address Geotargeting API is a great one, it is not a real database nevertheless its handy to create searches.

The Zipcode Database Project - A database that contains the fully city/condition in addition to coordinates for each zipcode in america. I have tried on the extender in several projects for handy city/condition zipcode searches.

reCAPTCHA... provides off-site CAPTCHA generation and validation.

The UC Irvine Machine Learning Repository - for data mining research

http://archive.ics.uci.edu/ml/