Nepomuk is interesting, but was difficult for me to get started with. It's part of KDE4, which is quite monolithic compared to anything else I use regularly and isn't very well packaged by my distro of choice.

kdebindings has Ruby bindings for Soprano (Nepomuk's triplestore interface), but I can't get their prerequisites to compile.

Ruby's DBUS bindings don't seem to behave the way the API documentation and tutorial say they do, but I managed to get something that runs.

It's a little bit sluggish, I'm not sure what to blame that on. Maybe I should use the Sesame backend instead of the Redland one? (Unfortunately Sesame is Java, which is another monolithic, self-contained software ecosystem.)