Vivisimo, a top-rated search engine that competes with Google for being #1, has some really cool indexing functionality that displays 'clustered results'. Clustered Results provide related terms & # of results, with drilldowns into a tree'd display.
There's a plugin for Firefox too.
It has indexed the 911 Commission Report as part of its service.
Another product they offer is a massive indexing of a company's databases, intranets, filesystems, etc.
There are some great demos for the medical industry here.
Plus you can search various Open Source repositories (Sourceforge, Freshmeat) which provides some amazing results.
How about Microsoft's Patent Portfolio (is double-click in there somewhere?)
Maybe RBC Financial for some good stock tips?
Definately a bookmark candidate.