Lucene is an indexing search engine. It's in Java, but it has ports to other languages including several for .NET.
One of those several, dotLucene looks very well maintained and some great demos appear on the site.
So how useful is it? Well, if you're a careful reader, you'll notice that on the openLucene site, there's a small list of applications that use the Lucene for .NET engine. One of those is the now very-well-known-since-microsoft-bought-them LookOut (email indexing addin for Outlook).
Looks like it's worth checking out.
[via Larkware]