With this post, we're kicking off a new occasional photo feature that will take you inside of the headquarters of today's top tech companies, from the big multinationals to the young startups of Silicon Valley. If you'd like to participate in this project, or want to suggest a company you think we should cover, email me at njackson[at]theatlantic[dot]com.

A free photo sharing application that has been called "the hottest iPhone startup right now," Instagram operates out of Twitter's old office at 164 South Park St. in the South Park neighborhood of San Francisco. The company, founded without a business model or any specific financing in mind, has quickly grown: It now has more than four million users sharing photos through the app and posting them to a variety of social networks such as Foursquare, Twitter and Posterous. It has just four employees.