YQL

The Yahoo! Query Language (YQL) web service allows to query web data in an SQL-like syntax, so you can do things like:

Get all cat pictures from flickr:

select * from flickr.photos.search where text='cat'

Get pictures from flickr that have interestingness starting with word Chinese:

select * from flickr.photos.interestingness where title like 'Chinese%'

Get sushi places in San Francisco that have a rating bigger than 4:

select * from local.search where query='sushi'
and location='san francisco, ca'
and Rating.AverageRating > 4.0

Get the profile of all my friends(my social connections):

select * from social.profile where guid IN
(select guid from social.connections where owner_guid=me)

It allows to combine several different sources in the same query, just like a SQL query that uses several tables, is possible to join results of GEOIP services with restaurant rating services, rss feeds, financial information, weather service, etc etc...

Nice one Yahoo!

Screencast: Introducing YQL

YQL site