maps.apple.com - see Colan's list
mapquest.com - I believe is only through Yext (starting separate thread to see if anyone knows a way to bypass Yext)
yellowpages.com - YP Network (also includes Superpages & Dexknows)
New advertising option:
A review of your product or service posted by a Sterling Sky employee. This will also be shared on the Sterling Sky & LSF Twitter accounts, our Facebook group, LinkedIn, and both newsletters.