Beagle is a cool OSS application written in C#. If I am correct, it currently runs on Linux using Gtk# and there is an ongoing effort to port it to Windows. Beagle is a desktop search tool. It indexes your data (doc, pdf, images, html,… files) and let’s you search them efficiently.
I really like the fact that Beagle integrates well with other desktop applications (mail, IM, photo albums,…) and that supports many file types. The coolest feature of Beagle is live incremental indexing of files. It indexes your files as you create them without having to rebuild its indexes. See it here in action.