I've wanted to build this for a while: favicon/icon as a service. Enter a domain, and get back an icon to represent the site. Could be a favicon, or a high-res application icon captured from meta tags. There are solutions to this now, but none of them are rock solid. You will end up pulling a hero image for twitter half the time. Params could include the size icon you are looking for, a good fallback, etc.

Imagine a product that deals with domains. You might want to show icons next to each domain in your product. You can build this yourself, as many do, but I think there is room for a "gravatar" style API that does this