When I’m explaining the semantic web to people, I start by saying that I think of the present web as one big global document, made by linking together pages on different servers. Similarly, the semantic web would link data from many different servers to make a global database.
That vision just got a step closer with Yahoo’s YQL, a kind of super-API which allows you to perform SQL-like queries across data from multiple sites. The tutorial on Net-tuts uses the example of taking the latest tweets from a group of Twitter accounts. You could substitute RSS for Twitter to make a news aggregator (not a hugely imaginative application, but one on my mind recently).