GNOME and Nepomuk.
Tracker is like Strigi+Nepomuk: the metadata store and access layer
is in the same application as the indexing tool. The tracker indexer
could , in theory, be used to feed data to Nepomuk - they both use
the ontologies that came out of the Nepomuk project (of which
Nepomuk-KDE is just an implementation) . Xesam was at one
time going to be the cross-desktop search interface, so user apps
wouldn't need to care if they searched Tracker or Nepomuk, but
it seems to have died (they also had ontologies competing with
Nepomuk's, but that was dropped even earlier).