Zillow has become the go-to site for consumers wanting an estimate of a home’s value or a list of what’s available for sale in a neighborhood. But this fall, the website will go beyond real estate listings and start to make offers on Denver-area homes. After launching Zillow Instant Offers in Phoenix and Las Vegas this spring, the Seattle real estate giant will expand the program to Atlanta in September and Denver before the end of the year. And there will be competition.