Beagle
Beagle is a desktop search tool. It can index various sources to allow fast searching for information. It currently supports searching the filesystem, email messages, instant messaging history, web browser history, RSS feeds and many other sources are being created and added. Since Beagle is aimed at the Gnome desktop, it can currently only read the email from Evolution, the browser history from Epiphany and the IM messages from GAIM, etc.
For searching the filesystem, Beagle supports getting extra info from certain filetypes like openoffice.org or Microsoft Word documents, HTML files, JPG images, music files (like mp3 and ogg) and many other types.
Beagle consists out of a daemon beagled which does the indexing of the files and other information and a GUI called best for searching. For storing information about when the a file was last indexed Beagled uses extended attributes (and falls back to a slower sqlite database backend when that's not available). It can also use inotify when that's available on the system to receive notification of when to index changed or new files.