One of the most sought after datasets in the world is Cities in the U.S. with polygons. I've been looking hard because I want to be able to click on a City name and present an outline of the City on a map. I finally gave up because it ain't out there, and created my own!
The current cool magic number is 36,654, which is how many Cities I've added to my database. Every Zip Code city has a city record with a polygon. It's better than the government's DB and the implications are immense.