Downtown San Francisco is ground zero for startups. It's also home to some of the most coveted real estate in the country. Space, as always, is at a premium. So when online reservation site OpenTable invited us to its 50,000-square-foot IRL digs, we couldn't resist. With spaces that pay homage to restaurant vibes, the two-and-a-half floors occupied by about 300 employees are designed for maximum collaboration.