Beagle is a desktop search tool that is very similar to Google Desktop Search and Spotlight (the search tool built in Mac OS X Tiger). It was originally built for Gnome, but can also run on Windows. Although developed in Mono (the open-source implementation of .NET), Beagle uses Lucene (a search engine written in Java).
Like Google Desktop Search and Spotlight, Beagle allows users to search files in different formats, mailboxes, and instant messaging logs. Searches are dynamic, which means that results are updated instantly as changes are made to the matching objects. This dynamism is what makes Beagle such an interesting application.
Nat Friedman has posted a few Flash demos of Beagle on his web site.