Folks interested in "open knowledge" should check out CKAN (Comprehensive Knowledge Archive Network)
CKAN is the Comprehensive Knowledge Archive Network, a registry of open knowledge packages and projects (and a few closed ones).
Less than two months ago, Merriam-Webster announced that some of its references would be available via an API. Now, competitor Cambridge University Press has followed suit with the Cambridge Dictionaries Online API. Currently, Cambridge offers five dictionaries through an API (i.e. Cambridge Advanced Learner's Dictionary, Cambridge Dictionary of American English, Cambridge Business English Dictionary, Cambridge Learner's English-Turkish Dictionary, and Cambridge Leaner's Dictionary).
Service discovery is not an easy task in today's Web. Discovering an API requires searching through a large number of services on the Internet and then reading pages of documentation to figure out how to use the ones that may match your application. This is the case in Programmable Web as well. The API directory shows over 5000 APIs which are manually categorized in over 50 service categories.