The desktop search program for GNOME (Beagle) is written in Mono. Last I checked it is a memory hog and is kind of unstable (I think I disabled it when it started causing my system to crash) which may or may not have to do with the Mono stuff. I do remember when it started out it was a real pain to install and get running because of all the Mono dependencies and DLL Hell (literally, there are .dll files). Interestingly, the Deskbar applet that you can use to query Beagle (which is a pretty decent UI and works fine) is written in Python, my favorite language. This also makes it brain-dead easy to write new search components.