I mention it only because he's done so much important work in regional and urban econonomics. Geography and Trade simply (and elegantly) explains why industries tend to concentrate in single cities or regions -- i.e., why agglomerations are important and why we see them everywhere.
Update:
Tyler Cowen describes this work:
I am most fond of Krugman's pieces on economic geography, in particular on cities and the economic rationales for clustering. He almost single-handedly resurrected the importance of "location theory," an all-important but previously neglected branch of economics. Here is the best summary piece of Krugman's work in this area. I believe this work will continue to rise in influence.
Here is urban economics savant Ed Glaeser praising Krugman's contributions to economic geography.
I do think Krugman is a political hack and wish he'd stick to economics in his New York Times column.
Update 2: Oh, and here's Krugman's essay on rent control, one of my favorites (either by Krugman or on rent control).
