Well, a file browser anyway. ChipBlog points to an implementation of tagging built right into the GNOME nautilus file browser. Not exactly revolutionary, but definately evolutionary, and an interesting way to address dealing with files and searching in the desktop.
/me waits for this feature to be announced for vista for 200
Update - It's been pointed out to me that tagging of sorts is in Vista's feature list already, I remember that now. It's almost true I am a grass roots yuppie vista hater, and I'm also strong enough to admit when I'm wrong :) However that said I don't think that the tagging in vista will be quite that sweet and easy. I can't find any screenshots of tagging in vista, other than this shot which has the "keywords" at the bottom of the bottom window. There's also a google hit on this page which talks about security concerns with metadata. Anyone have better references?
That said, the person who pointed out I was a yuppie also noted that leaftag through nautilus wasn't released either (however, with the 6 month GNOME relesae cycle, I have faith that that'll be out before Vista.... ).