A trend is popping up in retail and restaurants: pop-up sites that open for limited runs -- usually to big crowds. The low price of commercial real estate is helping facilitate the trend as many landlords are happy to have a tenant even for a short stint. The sites often generate big buzz and allow owners to test concepts or innovations. "I used to be in the music business and it's become almost like promoting a show. And people are excited by that," said one restaurateur.