Boston-area subscribers to Comcast Corp.’s Xfinity broadband Internet service now have a new way to get online. Comcast has set up dozens of Wi-Fi wireless Internet hotspots in Greater Boston, eastern Massachusetts and southern New Hampshire. All of them can be used by Xfinity broadband customers at no charge. Comcast already offers similar services in Philadelphia, as well as parts of New York, New Jersey, Maryland and Delaware. Users of Wi-Fi-enabled devices can access the free service by connecting to open hotspots named “xfinitywifi.” A map showing hotspot locations is available at www.xfinity.com/wifi.