How do content discovery engines, like Zemanta and Open Calais work?
Open Calais probably use language parsing technology and language statics to guess which words or phrases are Names, Places, Companies, etc. Then, it is just another step to do some kind of search for those entities and return meta data.
Zementa probably does something similar, but matches the phrases against meta-data attached to images in order to acquire related results.
It certainly isn't easy.
I was wondering how as semantic service like Open Calais figures out the names of companies, or people, tech concepts, keywords, etc. from a piece of text. Is it because they have a large database that they match the text against? 
How would a service like Zemanta know what images to suggest to a piece of text for instance?