adding categories to a large number of posts

This is more of a curiosity than anything…

One of my sites has 3000+ posts and due to my own laziness and a few other extenuating circumstances, none of those posts are categorized. Lately I’ve been thinking about organizing my posts in categories. As best as I can tell, the only way to do this is to manually go through and tag each category.

So I just wanted a little feedback: in your collective experiences, is there a different/better/easier way to add categories to a large quantity of posts?