Tagging online content with categories or keywords is a great idea.  I love GMail's use of labels instead of folders.  I tag all my Flickr photos six ways to Sunday. 

What I'm not so excited about are sites that encourage others to put public tags on my stuff.  Who are these people?  And no, stupid, that photo is not a parrot, it's a penguin.  I should know, I was there!

This sums it up quite nicely: