Boulder's startup scene has skyrocketed in the past several years. At monthly startup meetups, hundreds of people usually show up to check out what everyone else is working on.

It's not quite a Silicon Valley, and its score is probably exaggerated by the difficulty in defining a population area that's relevant to the startup scene. However, most startups in the scene are in Boulder proper, and most founders seem to live in the city as well. So the density is there, and it's quite easy to run into other startup people on the sidewalk or in coffee shops.