I've been remiss in not pointing out Maciej's work on the Semant-O-Matic. This remarkable little search engine uses a technique called "latent semantic indexing", which indexes word co-occurences. The result is that you can search on a concept that you put in as a search term, and it will return results that relate to that concept, even if your specific search term doesn't literally occur within those results.
It's already interesting to play with, but the implications of such a search being built on a larger scale are tantalizing.
